Again, can anybody give some advice for QUICKLY laying out a thick coat of gelcoat over a very large surface area such as a door panel? It seems like the brush tends to lay a thin coat. What kind of brushes are you all using for applying gelcoat?
Is it normal to mix up 4-5 batches of gelcoat before completely covering the work area? If I use a higher catalyzing percentage, I’ll probably have to mix up even more batches before the door is completely covered.
Ideally spaying the gel is best. You can get a cheaper gun with a bigger tip and thin the gel a bit. You can get a good thick even coat that way.
You could also thicken the gel a bit with a filler. With a thicker gel you can lay on thicker layers even with a brush.
If you get a good even layer of gel I’d wait overnight to start the layup. The gel will be mostly cured but the outer surface remains tacky unless wax is used. With the hard gel, just lay down a layer of resin and start the lay up beginning with some mat or veil.
If you want to cover a large area quickly - use a bigger brush! I prefer something like a varnish brush rather than a house painting brush as the varnish style brush has a much lower bristle density, meaning that you can load the brush with more gel coat (or resin as the case may be).
Edit >> If you use a varnish brush or any other type of cheap brush, be aware of the bristle lengths. You may need to trim the tip of the brush to get a more even length all over, otherwise if there’s a bunch of long bristles they will leave a kind of trench in your gel coat. Only found that out when I did a test part last week, didn’t seem to affect the final result though.
The brush will apply a coat thickness depending on the pressure you apply to the brush. Pushing into the surface more will give a thinner coat as the brush acts like a squeegee and pushes some gel coat in front of it. Less brush pressure and more frequently loading your brush will let you put down thick coats and work quicker.
It wouldn’t hurt to do a small test piece (just off a sheet or perspex/glass/etc.) before you next attempt on the doors, just so you can be sure that you have your brush application technique worked out.
Also I don’t think you mentioned, but are you using a brush gel coat or a spray one? They are usually made specifically for either brush or spray application. You can still brush a spray gel coat, but it will tend to go on thinner. No problem as long as you compensate for it - actually when I bought my last batch of gel coat the guy I was talking with said he uses brush gel coat for his parts (motorcycle/sidecar fairings etc.) because he doesn’t want the added weight of a brush gel coat that would go on thickly.
I would check your datasheet and do a separate test piece before proceeding with your doors. Test, test & test again… it’s the only way to be sure that your process will yield acceptable results.
Thanks! That is extremely helpful advice.
So I ordered another batch of gelcoat…this time from uscomposites.com. The guy I spoke with recommended rolling it on (instead of brushing it on) to cover the large surface area of my door.
Contrary to what some have said in this thread, the rep DID recommend applying a second coat of gelcoat approximately 2 hours after the first coat.
Anybody used a roller for gelcoat before?
I rolled whole boatmoulds in gelcoat 
(2 coats indeed)
At some wharfs I worked they liked to brush after rolling it on, for a smoother surface. Others didn’t, took too much time.
You can get a good result by rolling it, keep a brush nearby for corners, etc. and a thickness gauge to measure your gelcoat thickness.
(the coating of the inside of the hull was done mostly with a bucket,20 feet boat in 20 minutes…)
I’m interested in what the rep said about the gelcoat application method… what kind of thickness was he talking for the first and 2nd coats? I mean if you’re put down a thin coat first, I wouldn’t have thought the 2nd coat could make any difference since the first already has time to wrinkle up?
Also why 2 coats? Only if you’re having problems getting enough thickness in a single coat?
If I’m ever putting on a second coat of gelcoat I don’t do it until the next day…had too much gatoring trying to do it too quickly; however, now I spray my gelcoat with a dumpgun so 95% of the time I’m putting on one coat. Have had zero issues since I started using the dumpgun.
no one seemed to mention the pva… could that be an issue with the wax/gel coat? just a thought but i never used pva so idk…
The guy I spoke to was a rep at uscomposites and he seemed to enjoy saying other people don’t know what they’re talking about. Lol. I was calling to ask about making an epoxy mold (as recommended by the gentlemen above who made the cf Ferrari doors)…he acted like an epoxy mold was the worst idea ever and that poly was superior in every way.
He definitely agreed that the alligator skin was from too thin of a coat. And yes, the second coat should go on 12 or so hrs after the first per his recommendation. He suggested rolling to quickly roll out a thick coat on a large work surface.
I actually bought a used 25 gal (i believe) compressor to spray gel coat. The specs are right at 5 CFM at 90 psi (as recommended for my dump gun)…but I’m afraid that a project as big as a door is a little bit beyond what my compressor can handle.
Shouldn’t be. PVA is just another safety mechanism to ensure mold and plug don’t live happily ever after.
Jim, what would you use to thin the gelcoat…paint thinner? Laquer?
use styrene for thinning of styrene based products 
Sprayable gelcoats usually have acetone in them. The acetone evaporates as the gelcoat is flying to the mould. Spray from too close a distance, or in cold weather, and you trap the acetone, causing porosity.
Problem with styrene is that the residual styrene in the cured gelcoat increases, causing problems with yellowing in the sun.
Mostly sprayable gelcoat is a balance between thinner base resin, styrene and acetone. My advise is to buy spray version, instead of thinning your own.
As mentioned above I always use acetone to thin the gel for spraying. It’s almost 120 this weekend so you can imagine how fast acetone evaporates here.
Yeah, when its colder I’ll go to 2.5% MEKP and that seemed to be fine. I’ll brush one thick coat on later in the afternoon and then lay up earlier the next morning. So roughly 12 hours. Now that its summer time and temps have come up I may not wait quite that long.
Now I will say that I only pulled a few parts from the moulds I did this way so don’t know if it affects the longevity of the mould in any way yet.
Show me the surface of your door?
Have you usedvan areosol on any part of your door
That croc skin is solvent attack. Gelcoat dosent lift unless something underneath has lifted. It is a reaction you thecsurface of your door
My poly doors made from poly tooling
I sprayed approximately 5 coats of PVA on top of the waxed door…two or three days later I laid down the gel coat.
Great work! Do you have more step-by-step pictures of the process?
I’m definitely jealous.
Maybe the PVA is what’s giving me all the headache with creating the mold???
any updates on that project or the shrinkage issues discourage you?
Yeah, I got a bit discouraged but I will try again.
It’s winter 10 degrees here now…I’m waiting till late spring.