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Reapplying gelcoat

Sep 24, 2018
1,078
O'Day 25 Chicago
I've read multiple articles about refinishing gelcoat. Unfortunately they sometimes contradict each other so I was hoping someone could clear up some basic questions for me

  • I've read that gelcoat is simply pigmented resin yet I'm pretty sure I've seen it sold in a one part mixture
  • If gelcoat is not pigmented resin, what is the base that the pigment gets mixed into?
  • Can reasonably good results be achieved with a roller or brush such as the tip and roll method?
  • Can this be sprayed on without a spray booth or expensive safety equipment?
 
Oct 22, 2014
13,454
CAL 35 Cruiser moored EVERETT WA
I understand it is pigmented resin. I think it is a polyester resin. you use Methyl Ethyl Ketone Peroxide (MEKP) as the catalyst. You can make the color pretty much whatever you want by adjusting the pigment.
 
Sep 24, 2018
1,078
O'Day 25 Chicago
I have a friend who works at a yard. He's sent me a few pics from the fiberglass shop. The guy there starts out with a very small amount of base/resin and adds in drops of paint. Every drop is then marked on a piece of paper so when he gets it matched he can make a larger batch.

Wow MEKP has a lot of uses! Any idea how long it takes to cure?
 
May 17, 2004
2,835
Beneteau Oceanis 37 LE Havre de Grace
The guy there starts out with a very small amount of base/resin and adds in drops of paint. Every drop is then marked on a piece of paper so when he gets it matched he can make a larger batch.
Good video here of that type of color matching approach -

His process probably wouldn’t allow easy repeating to a second batch - you’d need to repeat the mix and match process again. But as long as you don’t catalyze the batch until you’re ready to apply it can be big enough for the size of repair he was working on.

The video after that one shows applying it to the repaired area.
 
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Jan 11, 2014
6,328
Sabre 362 113 Fair Haven, NY
Gelcoat is typically pigmented polyester resin. MEKP is the catalyst for the resin. Pigments come in a variety of forms, some are dyes (blues) some are ground powders (whites) and any combination of the the different types. While they may be chemically different gelcoat pigments are similar to paint pigments.

Depending on the area to be covered, gelcoat can be sprayed, rolled or brushed on. Larger areas will want to be sprayed, smaller areas will be brushed or rolled. They can not be rolled and tipped like paint because gelcoat is way too thick and viscous. Unless the gelcoat is being applied to a female mold, it will need to be sanded and buffed out after it cures.

Gelcoat (and polyester resins) will not fully cure until they are cut off from air. The resin will remain soft and sticky until the area is covered with wax or PVA. Small areas can be covered with a plastic film, however, in large areas this is cost prohibitive and difficult to get a close to smooth surface.

Gelcoat will cure fairly quickly in a warm environment depending on the amount of catalyst that is used. Follow the gelcoat manufacturer's recommendations.

Color matching gelcoat (or paint) is an art. There are standard color codes and cards that can be used to make the match. The cards will list the formula for the color on the card. To match the gelcoat, use the color cards to find the best match and then use the formula. Some companies will lend or rent you the cards. Pantone is one of the companies that standardizes the color formulas.
 
Dec 27, 2012
564
Precision Precision 28 St Augustine
I have sprayed gelcoat to repair the deck area of a O 22 I used to own. I thinned it with acetone and shot it through a harbor freight gun. There are 2 types of gelcoat. Your basic gelcoat stays tacky unless the air is kept off of it. The second is what I used. It can be referred to as finishing gelcoat or gelcoat with wax added. Once applied the wax works it’s way to the surface which keeps the air out and allows the gelcoat to dry.

The gelcoat laid nicely on the deck area due to the horizontal surface. The surface allowed it to spread and flatten. No wet sanding needed to smooth out. A vertical surface will probably need wetsanded to get rid of the orange peel effect.

For small repairs I have used one of those hardware store glass jarred disposable sprayers. Thinned in the same manor.
 
Aug 10, 2020
153
South Coast 22 Rocky Mount
dlochner did a good job of summarizing gelcoat.

do not reduce it with acetone. acetone breaks gelcoat down on a molecular level and also causes discoloration. reduce with styrene if necessary for spraying. If you use a gun with a 1.8mm or bigger, reduction should not be necessary.

as mentioned, gel only cures in absence of oxygen. so add a wax additive (we call it patch aid) or spray with pva when done to seal it off. There are also some high gloss additives you can put in it that seal it off and make it flow better. When used they will make it spray almost like enamel.

Avoid "putty repairs" where you thicken the gel coat with silica or cabosil. they make it brittle and the repair usually shrinks and cracks with time.

Our local supplier offers a color match for $40. I don't know if anyone else does or not. The company is Paints Plus out if Stover Missouri and they ship worldwide.

for factory match gelcoat, contact Spectrum. Very good products. very close color matches.

If matching yourself, get black, yellow oxide and red oxide toners. A very small amount of pigment goes a long long way. work slowly in the daylight and match small batches.

Any questions feel free to ask. I have been doing gel work for years.
 
Jan 11, 2014
6,328
Sabre 362 113 Fair Haven, NY
I prefer the gelcoat without wax. This allows for multiple coats without sanding or needing to remove the wax. Sometimes when the gelcoat cures, it shrinks a little or the repair may be in a place with thick gelcoat that is better repaired with several thin coats than one big blob. Once you are satisfied with the job spray a little wax or PVA over the surface and it will cure almost instantaneously. Spray car wax works.
 
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Sep 24, 2018
1,078
O'Day 25 Chicago
How many coats are usually used for a repair?

The gelcoat laid nicely on the deck area due to the horizontal surface. The surface allowed it to spread and flatten. No wet sanding needed to smooth out. A vertical surface will probably need wetsanded to get rid of the orange peel effect.
Would thicker paint minimize the texture on a vertical surface?
If matching yourself, get black, yellow oxide and red oxide toners. A very small amount of pigment goes a long long way. work slowly in the daylight and match small batches.
My friend at the yard was telling me that they had a boat in there that had a hint of green in the white paint. Crazy! He also said blue is a common color in white
Once you are satisfied with the job spray a little wax or PVA over the surface and it will cure almost instantaneously. Spray car wax works.
That's awesome that it cures almost instantly! I cant wait to give this a try! Any recomendations for paint guns?
 
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Aug 10, 2020
153
South Coast 22 Rocky Mount
Would thicker paint minimize the texture on a vertical surface?
blues and greens come into play on occasion. i listed your common toners. Use VERY SMALL AMOUNTS! look for a gun with a 1.8mm or larger tip. I use an old sata with 2.0 primer tip. keep in mind that it kicks off quickly, especially if you have a quantity in your paint cup! be ready to dump your extra and have acetone or lacquer thinner handy for quick clean up.
 
Dec 27, 2012
564
Precision Precision 28 St Augustine
I think I shot one heavy coat but I may of shot a 2nd before drying. Sorry I dont recall. Prior to application I repaired damaged area with a marine sandable filler.
 
Sep 24, 2018
1,078
O'Day 25 Chicago
You don't say! Wish I had known that a long time ago!
This technique sounds like it's super easy and flexible!

@dlochner Are there any additives in some waxes that should be avoided? I assume a quick wipe with a solvent will remove the wax? Do you have to wait to sand or re-wax or is it really 100% cured in minutes?