How to maintain
and repair
fiberglass on a boat

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This page updated: May 2011

SailNet - Sue and Larry's "Cleaning Fiberglass Hulls and Decks"

From article by Dan Spurr in May/June 2006 issue of Power Cruising magazine:

Difference between a polish and a wax, from rp260 on BoaterEd forum:
Polish is a substance that contains abrasives. The degree of abrasiveness depends on the product and its intended usage. Polish is used for restoring a finish and this is accomplished by having the abrasives remove the surface layer which has been weathered or has grime embedded in it.

Wax is a protective coat for the surface of the finish. ...

Some products are labeled as a cleaner/wax. These have a percentage of polish (usually very fine) incorporated in the wax, and are intended for minor surface rejuvenation.

If you have used a product and found a powdery residue on the removal rag or clinging to the surface, either the product had a polish in it, or it had fillers to make up volume (indicating a cheap product). A pure wax (or as pure as you can get) will leave no dust behind.

I only use separate polishes and waxes - never the combined products. Over the years, I have found that I get better results from both items that way.

Remember that any time you use a polish, be sure to follow up with a good wax as the surface will be smooth and clean, but unprotected from UV, contaminants (airborne and otherwise) and general grime.

From Don Casey reply to question, in 11/2006 issue of Sail magazine:
Automotive paint is harder than gelcoat, so the compounds that are formulated for automotive use are more aggressive than marine compounds and will cut the gelcoat faster. You have to be very careful to not remove more material than you had planned.
In my experience, an automotive wax will work as well as a wax that has a boat on its label, but I've found that polymer-based "polishes" seem to be more durable than traditional waxes. The reason is that a polymer polish bonds more tenaciously to the surface than the wax and also holds its gloss longer.

PoliGlow (polyurethane-based fiberglass polish; water-based blend of polymers. Minimum 5 coats.)
Vertglas (A copolymer coating, not a wax or polish. 6-8 coats. Sold at West Marine.)
Practical Sailor 2004 review of fiberglass restorers
Wax test article in 11/15/2004 issue of Practical Sailor

PoliGlow (800-922-5013):
Price: $55 plus S+H for 32 oz of PoliGlow and 32 oz of PoliPrep, enough for a 32-foot boat.

From John Schamante on the Morgan mailing list:
Used the stripper that PoliGlow sells. Really easy. In terms of work no worse than washing the hull or deck with a scrub. I used 400 and 600 grit wet sand paper to clean around deck fittings and any stains that may have been on the boat. The only problem is that when the deck gets this clean and shiny, boy, do you notice any imperfections you have. I am not sure if there is silicone in the product but I don't think so [confirmed by PoliGlow; no silicone]. Call the company; they have been very helpful. One thing I would note is don't put on more than recommended. It doesn't make a difference. Doing it in the direct sunlight was best; dried in less than 30 seconds and ready for the next coat. We did the hull in the early spring and I had some streaks that I had to smooth out. They tell you not to go over a coat until it dries, takes longer than 30 seconds when its colder and no sun which is when I did most of the hull. The reason for the number of coats first time is to make sure you have even coverage. I used more PoliGlow than necessary because I tried to cover everything 100 % with each coat. Not necessary. I would also suggest that after the initial deck cleaning if you don't put the PoliGlow on the same day, you take a wet cloth with the stripper on it and wipe down again just before you apply. You don't want any surface dirt.
From Irene Gale on the Morgan mailing list:
Read your details of prep and application of PoliGlow. You did not mention protecting non-skid and teak trim while doing so. We have a huge amount of non-skid, which is surrounded by narrow areas of smooth fiberglass. We could tape the teak, but I don't see how we could tape all that non-skid. Our cabin trunk is 90% non-skid, as well as the seating areas in the cockpit. Any suggestions?
From John Schamante on the Morgan mailing list:
I have I believe the same amount of non-skid areas. I did not tape. I used the hand chamois around the non-skid areas and was pretty careful. After I finished I took some of the stripper on a cloth and went around the deck where I got any on the non-skid. Worked fine.

From Howard Marsch of PoliGlow on the Morgan mailing list:
We have PoliGlow on the non-skid and we like it. It can be slippery if you are barefoot and the non-skid is wet. You have two options. First, if you don't like it on your non-skid, you can strip it off with the PoliPrep and have your fiberglass the way it was. Or, you can go over the area lightly with Soft Scrub. That is an abrasive cleaner that will not remove PoliGlow but will rough it enough so the area won't be as slippery. Your seal will still be there so you can keep your non-skid looking clean.

From Irene Gale on the Morgan mailing list:
Lady at PoliGlow had a great suggestion for application in small areas. Take small applicator and staple it to one of those small, foam, throw-away brushes.

From unknown:
I am not a polishing/buffing type of girl! ... my arms get too tired ... a 26 ft boat is 26 ft on both sides! I have used Poliglow for several years and I love the way my hull shines! There is a cleaner that also comes with the kit to help "unoxidize" the hull. After I have cleaned the hull, I apply 6 coats of the Poliglow on each side ... the stuff dries quickly so as soon as you're done at one end of the boat, the other end is dry ... however, I have deviated a bit from the directions ... there is a chamois-type roller included to which you put the Poliglow on and then apply to your hull. I have found that the Poliglow tends to roll off, so I put the Poliglow in a spray bottle and sprayed it on the hull and wiped it in as I went down the side of the hull ... worked so much better and you don't waste any of the product ... you would not want to do this when it's too windy however. The stuff is great and the hull looks so good. You're suppose to apply it yearly ... 3-6 coats depending on how the hull looks. I think you'll like the ease, convenience and results.

From Capt. Rick Mauk on the Morgan mailing list:
I used it and it is an amazing product. My boat feels like it has a layer of plastic on it now. Before it was dull and made you itch. It has a very nice reflective finish. I love watching the water ripple reflections on her topsides now.

The product was very easy to apply.

From 4/15/2000 issue of Practical Sailor:
[PoliGlow and others] work, as long as your expectations are realistic. They will definitely make a dull finish shine and - if you've been thorough about compounding and/or polishing - you can get your boat fairly close to a new boat appearance ...

... should provide reasonable gloss for a season in almost any climate. You'll have to apply three maintenance coats a year ...

Fiberglass "restorers" are best thought of as remedies for weathered hulls rather than preventatives. If your boat is already shiny, just wax it. And if it's really gone, paint it.

From Gerry LaBrie on SailNet's Gulfstar mailing list:
I applied PoliGlow to the topsides and deck three months ago. This is a 2 part system (PoliPrep and PoliGlow) that completely cleans and polishes the fiberglass without the pain of buffing (you just rub the prep on and apply the Glow with a chamois). This is great stuff, easy to apply and looks great for at least a year (so I'm told by those in our marina who have used it for 3 years now).

From Loyd Tyler on the SailNet liveaboard-list:
Poli Glow remover alternative:

The product of choice for me is liquid Zud.
I have used it, but never purchased from the link. I can buy it at my local boatyard. My boatyard uses it to prep a boat for Poli Glow and to take it off. Apply with a 3M Scotch Brite pad after spraying the hull with water. Work on an area about 4' long at a time. I tried Poli Prep and it did not work as well as Zud. Give it a try, it's cheap. Works really well to remove rust stains on metal and fiberglass. Great stuff.

From Loyd Tyler on Latitudes and Attitudes Cruisers Forum:
Original gelcoat was tired. And I was not looking forward to a compound/wax job. I like technology! I found Poli Glow. This stuff is very easy to work with.

I did not like their Poli Prep solution as well as this stuff that I found at my local yard ... Zud. Zud will remove stains, including rust at the exhaust much easier than anything else I tried. Comes in a liquid and a powder. I like the liquid. Put some on a nylon pad from the dollar store, and wipe it on a wetted surface. Wait a minute or two, then go back over it with the pad to remove the oxidation.

Preparation is the key. You must remove the oxidation or your finished look will be spotty. For tough stains, I used a Scotch Brite pad and Zud. I had a perfectly clean topside with only one bottle for a 30' boat after about 16 hours of light scrubbing.

... I lost count how many coats I wiped on of Poli Glow, I think it was six coats. I will reapply another coat of Poli Glow this summer to maintain it. For the non-skid, I use the liquid Zud, and no wax or Poli Glow. I leave it plain but very clean.

Paraphrased from Epinions:
Zud contains oxalic acid plus strong scouring abrasives.
Bar Keeper's Friend contains oxalic acid plus citric acid plus much gentler abrasives.
Cleaners such as Ajax and Comet rely more heavily on chlorine bleach, powdered detergent and moderate abrasives.
Bon Ami combines a biodegradable detergent and mild abrasives.
So Zud is the most powerful, for the toughest scouring jobs.

From Scorpio on BoaterEd:
[PoliGlow] sounds like another product called Vert-Glass [Vertglas ?] by another maker. Basically these are acrylics like acrylic floor "wax". I've used Vert-Glass and like the results. I got two seasons out of the stuff in salt water and this year I'll strip it all off and do it again. On horizontal surfaces, I noticed it started to flake off toward the end of last season. Like others said, preparation is 95% of the work. The shine is awesome and people cannot believe I didn't spend days with a buffer and fine sandpaper. Does a very nice job on older oxidized gelcoats.

From Scorpio on BoaterEd:
I've used Vertglas for four seasons and like the results very much. Some people hate it as eventually you have to remove it and start over, but in the meantime, it is very easy to maintain. Nothing is forever and you either wax often or strip poly coatings every four or five years.

Removal is not that bad, just use a commercial floor wax remover and it will come off. I used a lot of paper towels and rags and some laquer thinner and had it stripped in one afternoon (26' boat). The laquer thinner was needed to remove the glue from my old stripes.

The shine these types of materials produce is beyond what wax will give and it stays that way for at least an entire season (I'm in the northeast). I've had people tell me the boat looks brand new (1982 Searay).

The key to getting good results is to follow the directions as to preparation and application. Just like painting, preparation is important. Don't skimp on the number of coats, more is better. I use a 10" applicator and it makes the hull sides go quick. I start at the stern and work forward and when I get to the bow, I go back and apply another coat. Thin coats are the goal, no runs that way. The first two to three coats will have you wondering what you're doing because it looks smeared, but the shine builds with the coats, and after about five coats it looks great. I restriped my boat last season and stripped the old Vertglas at that time so I put ten coats on and it looked like new. I can do the entire boat in a full day. This year I'll wash the boat and apply about two maintenance coats and I'm ready to launch.

I would highly recommend this for an older boat especially one with a dull oxidized hull. Newer (read shiny) boats will not benefit as much as older boats. This material works especially well on dark colors and stripes.

PS I got a quart bottle as the kit was a bit shy for my boat, also got the 10" applicator from Lovett Marine at the same time. They will sell it direct to you.
From Jeremy on BoaterEd:
I second the Vertglas for older heavily oxidized gelcoat. It does an amazing job in restoring the color and shine. The only downside is that you will need to strip it and redo it in about 4-5 years. A small price to pay compared to the fantastic results you'll get.

From Don Casey reply to question, in 11/2006 issue of Sail magazine:
NewGlass2 will restore the gloss and sometimes the color to faded gelcoat, but it is not an alternative to polishing. Rather, the product is a water-based emulsion of acrylic resin that dries to a smooth, tough film. But there is no free lunch; if you want new-boat gloss, you have to remove all the surface oxidation. That means polish-compound-polish or sand-compound-polish, depending on how weathered your gelcoat is. If you already have a clean, polished surface, you should be pleased with the results of a NewGlass2 application. But plan to renew the coating at least annually.

To remove acrylic polymer gelcoat coatings: TSRW QuickStrip.

From Gregory Saracco on BoaterEd:
Used Poliglow on an old Searay once. It looked good for a couple of months, but terrible once it started to peel off. A real bear to get completely off. I would think waxes would speed up the deterioration of the Poliglow. I'd never use it again unless compounding with a rotary buffer wouldn't bring the gloss back.


The problem I had with it was that it didn't hold up well to washing. I wash my boat at least weekly with boat soap. The Poliglow won't hold up well to that. I can't stand a dirty boat though so this product isn't for me. If you're into shortcuts, go for it.
From CaptKen on BoaterEd:
Watched them demo the Poliglow down at the Miami Boat show a few years back. The following week I ordered a case of the stuff. Be sure to get the kit with the cleaner. I used the first kit on my old 75 Chris Craft and made the blue hull look new. You have to be sure to apply several coats to get any life out of it. When it does start looking bad, you have to use the cleaner to remove the old coat before re-applying. An easier way is to wet-sand the boat and have someone spray clear-coat on the hull. Just as good and lasts for years. Same principle.
From Delaware Jim on BoaterEd 3/2007:
I used PoliGlow on my faded hull (1984 vintage) three years ago. It did improve the look significantly and have not had any peeling. Stayed good looking for 2 years and put another two coats on last winter. It does put a bit of "yellow" into the white hull, but it is not unpleasant at all. Beats the "faded washed out" appearance I previously had, and it is not too tough to apply - warning - follow the instructions precisely. About the third coat you wonder if you've made a big mess, but by coat 5 it starts looking good. My 36' hull took about a day and a half to do (6 coats).
From Scorpio on BoaterEd:
I've been using a similar product called VertGlass and have had good results with it also. Follow the directions carefully. Just like painting, preparation is the most important step.

Re: Peeling: I don't know much about polyglow but with Vertglass, you have to use a soap with an acidic pH. Highly alkaline cleaners like dish soap and black streak removers will cause the material to yellow and/or peel.
From Marc on BoaterEd:
I've used Poliglow on 2 boats - one a blue hull and another a white hull. Product looked great on both. Each spring I either recoat the entire hull or do touch ups. To touch up, wash the hull and then simply spot-apply more coats of poliglow until the touched up areas blend into the rest of the hull.

The stuff is easy to work with. I can easily apply 4 coats to an entire 26' boat in about 4 hours.

Problem noted above about not standing up to washing is correct. My transom gets a coating of soot that I periodically wash off. Last year I was using undiluted Simple Green. It took the soot off as well as some of the poliglow finish. As far as the rest of the boat, I find I do not have to wash it nearly as often (as compared to a wax-coated hull) since the poliglow creates such a slick finish and most dirt washes off with pressure from the dock hose.
From Carver370 on BoaterEd:
I have been the vertiglass route and I can tell you also that the product does not like washings of bird droppings. Vertiglass was probably the hardest thing to get off my boat once on, their remover didn't work at all basically and I ended up going to the local marine store and trying to find things to take it off. I ended up with "fiberglass solvent remover/dewaxer" by Interlux. In MY opinion, once you use these on your boat, you're committed to it for life, I NEVER want to do what I had to go through again, luckily it wasn't on my boat.
From Rommer on BoaterEd:
Poliglow, glorified acrylic floor wax at 10 times the cost!
From Mike F on BoaterEd:
Rommer is right on! Tried it on my previous vessel (dark blue stripes) and it looked terrible. It was also miserable to remove. Found later it is basically the acrylic floor coating as Rommer mentioned.
From Scorpio on BoaterEd:
I'm on my fourth season with this particular application and have not had any problems. I apply four or five maintenance coats per year, and wash with their soap after every outing. I will say that the flat deck on the bow starts to look a bit gray by end of season, but I just strip it in the spring and reapply six coats and I'm good for another season. I have not had a wax last that long on anything.

As for stripping it off, Rommer is correct, it is basically acrylic floor coating, so it follows that acrylic floor stripper should remove it (and it does) with very little effort and at a reasonable cost. Just visit a commercial janitorail supply company and get a bucket of the stuff cheap.
From Stephen on BoaterEd:
I used the Poliglow product for a few years and went back to wax. ... In general I found it was unable to hold a shine anywhere there was sea spray, like in the bow. Removal of the product is somewhat labor intensive, so be sure you want to do this. ...

From wax test article in 11/15/2004 issue of Practical Sailor:
After a year of exposure, Poli Glow was rated "excellent", almost as good as the best paste waxes. But PS prefers paste wax because of fears Poli Glow and similar products might discolor and deteriorate over time, or be difficult to remove.

From Robert Stuke Jr:
Clean off the stains with CLR or something, scrub the surface with white pads, and then a few coats of PoliGlow. That would keep it for 6-8 months before needing another application of PoliGlow.

I use the Scotch-Brite™ Light Duty Cleansing Pad 98 to get the loose surface stuff and chalk off the surface, and then hit it with PoliGlow.

I think it is an awesome product. But as with anything else, prep is the key. Some people talk about 400/600 grit wet/dry sandpaper. I used the Scotch-Brite pads because they were easier to hold, lasted tons longer, and I got better feedback as I was cleaning the fiberglass.

From Charles P. Cohen on the Morgan mailing list:
My fiberglass expert has very nasty words about any wax that contains silicones. They work really well -- but when it's time to re-do the gelcoat, or repair the underlying glass, they are very difficult to completely remove. Silicone residue causes paint to "fish-eye".

From John Dunsmoor:
Do not use any product that contains either Teflon or silicone; most of these [non-traditional] products do. If you get either of these compounds on the topside, paint will not stick, neither will gelcoat.

If you want to use these kinds of products make sure you have completed your painting repair cycle.

From Gary Elder:
> The boat's gelcoat is chalky and very neglected.
> Various dirt and stains that can't be scrubbed off.
> Local expert recommends wet-sanding 600 grit
> by hand, then waxing. I'm thinking of
> wet-sanding 600 grit by hand, then PoliGlow.

If it needs 600 grit, I would follow that with rubbing compound, then Finesse-It II, then carnuba wax. You probably know the theory about sanding ... You know, after you make big scratches with the coarse paper, follow up with ever smaller and smaller scratches until you are using wax. I am not a fan of hand-sanding unless it is necessary. It is dangerous [risk of electric shock], but some people wet sand using an electric orbital sander.
More from Gary Elder:
Now that I have seen "Magnolia", I need to re-group concerning your questions about deck waxing.

Your deck seems to be in decent condition except where the gelcoat has either been worn away or chipped. I would not sand it yet. It does have a finite thickness, and once it's gone, well, it's gone. I would pick a bad spot that is still fully covered w/ gelcoat, and try a COLOR RESTORER, using a low speed orbital POLISHER. If that gives you a satisfactory result, I would then apply a carnuba paste wax using a low speed orbital POLISHER. If the result of the color restorer is not satisfactory, I would consider using a fine rubbing compound using a low speed orbital POLISHER followed by carnuba wax. The CAPS are there to prevent confusion ... Color Restorer is a type of product that is a little less aggressive than rubbing compound. Polisher is in caps so it doesn't get confused w/ an orbital sander.

From Rod 5/2011:
38 year old gel coat with no wax tells you something, somewhere down the road. If you neglect a painted hull you're looking at a mess everyday until you spend the big bucks to repaint, and you must repaint. Once a painted hull, always a painted hull, no going back.

You know how it goes, you get your hull painted or machine-polished, and you're so proud of the fine-looking hull that you tend to want to do regular maintenance for a while, but that may fade over the years, that's when I am sooo glad I didn't paint my hull because, I may muster the will to do a little polishing, but not the expense to repaint.

Now about Poli Glow, it is a coating, not a real durable coating, but a real pretty shiny coating. It can not be touched by fenders, lines, your dinghy or anything that can scuff. It will look like hell in those places, it has to be cleaned at the waterline regularly or it will discolor like ring around the collar. And soon the ring will not come clean; hard scrubbing will remove it, leaving a patched-up look on the hull.

There ain't no easy way to have that "bristol" looking boat without many hours of hard work, and that's a fact. But the dude with a gelcoat hull can always polish the old girl up, if the need comes along, just for pride, or maybe, to make it look good if it is time to sell.

Back to your original question, no, [you don't have to wax after polishing gelcoat], but when you spend some bucks on your hull and get it all shiny and looking good you'll be so proud, who knows, you may turn into a detailing freak, spending hours over the side in your dinghy working on that blemish you just noticed. But yes a little Collinite polish can't hurt, but if you decide next year not to polish it don't matter, she just won't be as shiny. After all she has gone 38 years and you can still make her look good with just some Starbrite Hull Cleaner. That stuff is great.

Restoring badly oxidized gelcoat,
summarized from article by Jan Mundy in issue 2002 #1 of DIY Boat Owner magazine:
  1. Wash to remove contaminants.
    Use boat soap and a wash mitt or abrasive sponge.
    If water beads, wipe with solvent to remove silicone, wax and glaze buildup.
    At end, water sprayed on should wet out instead of beading up.
  2. Repair oxidization (which is a bumpy surface) by either:
    • Filling cavities with wax or polish, or
    • Taking down high spots by compounding or wet-sanding, and then filling/glazing.
      Compounding and glazing don't like heat.
      After filling/glazing, wait a couple of days before waxing.
  3. Protect the repaired surface: wax it.
    Want moderate temperatures while waxing, then several days of same or higher temperatures to make the wax bond to the surface.
  4. Protect the wax, which gets degraded by sunlight and water-drops that focus sunlight on spots.
    Use spray-on protectant such as Clean & Shine.

Restoring gelcoat, from article by William Burr in 4/2004 issue of Cruising World magazine:

From Irwinsailor, copied on Latitudes and Attitudes Cruisers Forum:
... the finish on the hull was very dull, it looked so bad that I thought paint would be the only thing we could do. A friend told me what to do to bring it back. This sounds like an ad; it is not.

I washed the boat. I needed to buy a polisher and compounds for about $300. First I applied 3m super duty rubbing compound #05954 $14 QT and took it off with the polisher using a 3m perfect-it foam pad #05723 $10 each. The boat looked so good at this point I did not think step 2 would be needed.

I did a test area for step 2 and found that this step improved the finish as much as step 1 did. Step 2 is to apply 3m finesse-it II #05928 $24 QT and polish it off using 3m perfect-it #05725 $10 each. Unbelievable!

Then #3 wax with Collinite's #845 insulator wax $14 PT. WOW! I can not believe how many people come up and ask me who I had paint the boat. When I tell them that I just polished it they are stunned.

Our boat is 61' and I only used part of one bottle of each product.

From Jim on The Live-Aboard List:
Re: Power-washing gelcoat:

I use a small electric 1200 PSI washer routinely to clean off bird droppings and such. Cleans pretty well. I painted my deck last year, and there are a couple of spots that apparently I didn't prep well enough and the paint came up.

Remember -- the 1200 PSI is at the nozzle; it falls off quickly with distance.

From BoaterEd forum:
> at what point does one decide the fiberglass gelcoat
> is beyond repair and paint with imron is necessary?

From Bruce / pdecat on BoaterEd forum:
IMO paint is better than gel coat. If it wasn't so expensive mine would have been painted by now.
From Cor / sabrejocky on BoaterEd forum:
I'll take Awlgrip over gel coat any day, gel coat finish is just being cheap on part of the builder. Once painted all it takes to clean the boat is a little soap and water. And non-skid that is painted with Awlgrip does not get hot in the summer!
From Gregory S on BoaterEd forum:
One problem with painted boats up north is that they are difficult to shrinkwrap in winter, can't get that stuff anywhere near a painted surface.
From George / caltexfla on BoaterEd forum:
The modern two-part paints, Imron (even the old stuff for that matter), Awlgrip, Alexseal, Interlux, et al last a long long time with proper care. They can be buffed, but not as aggressively as gelcoat, nor do they need to be. A light glazing material like 3M Finesse-it II is usually all that is required, if anything, followed up by a sealer/polish like Rejex (my favorite), or ProPolish, AwlCare, Starbrite PTEF, etc (no wax!). It's the only way to go in my opinion. Once you have the above in place, you just wash with water or a very very mild soap (something like AwlWash) and water solution. The paint manufacturers have their suggestions online in most cases.

From Ghost on BoaterEd forum:
> ... transom storage doors, one of which was damaged and cracked ...
> I repaired it with fiberglass and sanded the whole surface down ...
> I called Spectrum paint to order the matching gelcoat ...

You can't really trust that the "off the shelf" color is going to be correct for your boat. Your gel is changing colors the moment after it is laid down, slowly but it changes. On my 30-year-old boat, with an off-white gel color, last year I made gel repairs all around the boat. We ended up using 3 different colors of the "same" color for it to ultimately match at different points around the boat. It amazed me how before we would spray, it would all look the same, then once the gel was on, you could see distinct color differences. The differences of course are due to which sides of the boat get the prevailing sunshine and the associated rates of yellowing over time.

Also, since I have both companies, Spectrum and Gel Coat Products, in my back yard, I can tell you that just because it's a "stock" color does not mean that they might not very well be mixing it up fresh when you order (especially for a gallon).

Don't try to "paint" gel. Roll/tip may work with paint, but for gel it will cause more work. Spraying, even bad spraying technique, will result in less work than brushing. A decent sprayer and good technique will result in the least work, but I learned last year that even a disposable preval sprayer will leave you MUCH further ahead than any brushing technique I've ever been capable of. All you need is to thin with either acetone or preferably styrene. You will never get a "finished" quality result from either spray or brushing as this is not like the process where gel is sprayed first into a mold. So the final step is going to be finish sanding and then buffing/polishing. If you spray well you can START with something like 400 or even 600 grit. For brushing I've had to start somewhere around 200 grit.
From Rub Rails on BoaterEd forum:
I did gelcoat for 25 years and Ghost is absolutely correct. I always mixed colors on site and yes, one side might not match the other. I usually started with white base from scratch but if factory color was available, I would sometimes buy it but always have to doctor it some to match.

One thing that changed the last fifteen years of that career was a product called Duratech Clear Gloss Additive. I gave up Styrene and Acetone. The gloss additive cuts it 50%. If additional thinning is needed, use Acrylic Lacquer Thinner. The gloss additive converts the chemical cure to a tack-free air cure, eliminating the wax additive or PVA overcoat. It cures quicker, tack free, and also flows out more, like paint, reducing wet sand and buff time/effort. If you use this, make sure gelcoat contains no wax additive. Most gelcoat does not contain wax additive unless requested. Duratech will not cure with wax additive. This is best for use by professional as quarts are hard to find and shelf life is limited.
From PascalG on BoaterEd forum:
Gel coat is NOT paint, it's a "resin", you can't roll n tip it! Has to be sprayed then it has to be sanded to remove the orange peel, then buffed and polished.
From Ghost on BoaterEd forum:
Another tip, actually more like a "danger Will Robinson" is when you spray the gel on, make absolutely sure that you don't spray onto any "shiny" un-prepped surrounding gel. Obviously you need to blend the new gel into the old, but what is not as obvious is that the VERY lightly edges of the blend must NOT touch any unsanded gel. Prep the whole area that will be blended with 400 grit. If you happen to blend a little onto the "shiny" un-prepped gel, the color WILL NOT match and for reasons my mechanical mind can not seem to comprehend, just sanding it off is not as easy as you might think. On the other hand, if you blend/spray onto the sanded you have a hard time even seeing color differences. It's much more dramatic difference between the two cases than I would have guessed.