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Applying Gelcoat with a 4" foam roller

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Steve Blackburn
Master Marine Consultant

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Canada
1091 Posts

Initially Posted - 12/03/2007 : 17:06:19

This is another "don't do what I did" thread.

Recently I fixed my rudder for minor imperfections, it had a big bump to one side painted over (just a shotty previous repair job) and some chips. I sanded it down to the original gelcoat and slightly beyond in certain areas. So I had the great idea of rolling 3 coats of Gelcoat with a 4" foam roller as if it was paint then wet sand. Looked pretty good until I noticed that I rolled in tiny bubbles. You can't see the bubbles fromt the superficial coat they are hidden right underneath and cure that way. Sanding it down reveals multiple tiny holes here and there (like an Aero chocolat bar). Last night I tried the paint brush technique and once I sanded, no bubbles!

A paint brush leaves bristle marks that must be sanded down and is difficult to apply a regular thickness. Sanding down a paint brush application also means that you must sand down to the lowest point of any bristle which leaves a very thin coat. A roller leaves an orange peel, regular thickness but bubbles. Tonight I will try a foam brush using long strokes for eveness. I'm hoping the foam brush result will require less sanding leaving behind a thicker coating.

Steve Blackburn, Calgary, AB
C250WB - 1999 - Hull 396

Edited by - Steve Blackburn on 12/03/2007 17:14:56

Happy D
Admiral

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921 Posts

Response Posted - 12/03/2007 : 18:24:09

Spray it with a [url="http://www.prevalspraygun.com/"]Prevall[/url] sprayer and thin it with acetone.
You're not supposed to use acetone to thin it with, but for a small rudder job, it will probably be OK. Mix a little and spray it. Let the solvent evaporate and spray a little more.
Spray only what you can in 20 minutes, then clean the gun. Mix up a liitle more and spray a second coat. Make sure you leave plenty of time for the solvent to flash off before spraying the next coat.
Don't let it kick in the gun. Have a spare power head on hand just in case. They're about $3.00 here. I get them at Ace Hardware.
You can leave the coats on you already applied, just dry sand with 220 until flat and spray. It'll look great.
Are you using Duratec or patch aide?
Dan

Edited by - Happy D on 12/03/2007 18:25:21

Steve Blackburn
Master Marine Consultant

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Canada
1091 Posts

Response Posted - 12/03/2007 : 19:26:45

That's pretty neat. What is the mix ratio of acetone : gelcoat you use?

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Ericson33
Admiral

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USA
892 Posts

Response Posted - 12/03/2007 : 22:12:57

you can use a roller with the gelcoat, it sounds like you got into a hurry, the real trick is to spray the gelcoat with pva right after you finish applying the gelcoat, you also need to prep the area with sandpaper, then clean it several times with the asetone.

I used a 30 to 1 ratio on the PVa, I had no luck with the peval sprayers, too much splatter, I used a small air gun for touchup paint jobs, you can also apply several coats, then sand with 200, 400 wet, spray black laquer, very light, then sand with a long board to see the lows in the gelcoat, apply another coat, sand again with 400 wet, then finish with 600 wet.

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Steve Blackburn
Master Marine Consultant

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Canada
1091 Posts

Response Posted - 12/04/2007 : 03:10:22

I don't think I need to apply PVA since my marine grade Gelcoat is prewaxed. Cures in air without the need to seal it with PVA.

How thick in milimeters (mm) does the gelcoat need to be and how many coats does this equivalate to? In certain areas seems I sanded too much and the older yellowish gelcoat is showing through. I put 3 roller coats. Doesn't feel like I'm taking that much off either, as soon as it's smooth I stop sanding the area. I use 220 wet sandpaper.

Tonight I used a foam brush and I have no bubbles. Does a real nice job and requires far less sanding. You need to change the brush often as they degrade and make for a less smooth finish after 5 minutes of use. I think with 2-3 coats with a foam brush and light sanding in between that I will have no problems.

Still I'm interested in learning more tips and guidelines if you can offer some.

Edited by - Steve Blackburn on 12/04/2007 03:17:22

dlucier
Master Marine Consultant

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Virgin Islands (United Kingdom)
7583 Posts

Response Posted - 12/04/2007 : 07:53:59

When I've done small gelcoat repairs, I've applied the gelcoat paste with a spreader then covered the area with plastic wrap. The repaired areas came out nice and smooth, and after a little sanding, the repair was virtually imperceptible.

[url="http://www.boatus.com/boattech/casey/02.htm"]Gelcoat Repair[/url]

Edited by - on

Happy D
Admiral

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921 Posts

Response Posted - 12/04/2007 : 07:58:50

The ratio to thin with acetone varies. I use gelcoat mixed 50/50 with Duratec High Gloss Additive. That thins the gelcoat enough that it can be sprayed with my big gun. I have a spray gun, gravity feed, 2.5mm nozzle, Non-HVLP for blowing the gelcoat on.
When I did a small repair on the interior gelcoat, I used a Preval sprayer and thinned the gelcoat a little so it would come out of the Preval a little better. Maybe 5–10% with acetone.

I buy my gelcoat from Express Composites in Minneapolis.

Gelcoat viscosity can vary from 1000 cps to way over 2000. I cannot find a good viscosity number for gelcoat. Everybody's gelcoat is different.
Minicraft sells a gelcoat that has their proprietary additive mixed right in that adds gloss and hardens the gelcoat. You spray it right out of the can.
The gelcoat I buy is thick. A wooden stir stick will stand up in it. Once it's thinned with Duratec, it much less viscous. The viscosity of Duratec is 100 cps so you can see it thins the gelcoat a lot when you mix it in.

The Duratec is made to modify the gelcoat so it can be sprayed and it causes non-waxed gelcoat to harden on it's own. Duratec suspends the shine throughout the thickness of the gelcoat. As you keep sanding, it uncovers more shiny gelcoat, if that makes sense. I think patch aid just thins the gelcoat but I have no experience using it. Don’t know anything about it.
Gelcoat shines by touching a shiny surface. The gelcoat on a brand new boat shines like the dickens, on the surface. But sand that surface away, and there’s all those little air bubbles from spraying. It’s only gorgeous on the surface. Beauty is only skin deep applies to boats.
Best thing would be to spray the gelcoat and then cover it with a perfectly smooth piece of plastic sheet. Lie that on so the wet gelcoat is touching the surface 100% and let that cure. You’d be done and it would be gorgeous. You could squeegee out the air bubbles and see what was going on as you did it. It’d be tough finding a perfectly smooth piece of plastic. Lay the rudder down and do one side at a time maybe. Any little wave in the plastic will come out in the gelcoat surface.
As far as thickness is concerned, I see the number 25 mils a lot. Gelcoat has around 40% styrene monomer. The styrene monomer is a solvent that evaporates. So the gelcoat thins about that much when it cures so I’ve found in my very limited use of this stuff.
The idea is to get it on flat and sand a little. If only it was that easy. Have fun.

Edited by - on

Steve Blackburn
Master Marine Consultant

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Canada
1091 Posts

Response Posted - 12/04/2007 : 10:30:57

Thanks for the tips guys. I do and did use the plastic sheet method on scratches and gouges on the boat. Even with your nose straight up to it one cannot tell where the repair has been done. However this rudder is a different animal altogether with it's multiple curves.

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