Polyurethane primer and S3 Quick Fair

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wdr99
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Polyurethane primer and S3 Quick Fair

Post by wdr99 »

Hi all,

I'm fairing the bottom of my FS14. I've applied at least two coats of System Three Quick Fair and a couple additional coats of epoxy, all sanded with 120 grit. I applied a single coat of Petit Ez prime polyurethane primer over a small part of the hull and I'm noticing a bit of the 12oz fabric weave through the primer. I'm used to 6oz glass so this 12 oz stuff is pretty different to work with. I'm using the EZ Poxy stuff because I've used it before, it's one part, and I can get it locally.

Should I prime the whole boat and go back with some more quick fair? I'm not sure if Quick Fair would be appropriate over the one-part polyurethane. Or should I wait to prime until the hull is as perfect as I can stand? Or is there another plan?

Thanks!
Bill

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Re: Polyurethane primer and S3 Quick Fair

Post by Jaysen »

The only thing that should go over primer is paint. Finish the fairing then prime.
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Re: Polyurethane primer and S3 Quick Fair

Post by Dr Pete »

Jaysen said it but to make it clear….the weave needs to be filled. Thickened epoxy or fairing mix. Then sanded/faired smooth. Now the surface is ready for priming. Depending on what you faired with and thickness of the priming coat you might see weave. You should not be able to feel it.

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Re: Polyurethane primer and S3 Quick Fair

Post by fallguy1000 »

Sand your primer coat with 120 grt and get out some fairing compound...butter up that weave; you missed a step is all...

after fairing is applied; typically long board sanding on a 45 degree angle is next; followed by more fairing
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Re: Polyurethane primer and S3 Quick Fair

Post by TomW1 »

12oz is hard to fill I have usually used 3-4 coats to fully fill the weave and get a smooth bottom with no high or low spots, which is another reason to use the fairing compound. This will make your bottom smooth. Use eighter a fast drying spray paint like Krylon in block and make speckes all over or dust all over with grafite and sand now before you add the next layer of QuikFair.

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Re: Polyurethane primer and S3 Quick Fair

Post by joe2700 »

It's pretty common to do a bit of additional fairing over primer after the primer shows you imperfections you missed before. When both are epoxy products there is no issue with that, with a one part poly paint I'm not sure, probably safer to sand off the primer.

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Re: Polyurethane primer and S3 Quick Fair

Post by wdr99 »

Thanks everyone!
I am curious specifically about epoxy fairing over the polyurethane, I don’t know if that’s ok. I’m guessing it’s not ideal. It’d be handy because it certainly highlights any imperfection.

I’ll sand off the primer, it was only a 1.5 square feet of the bottom, and do more filling and fairing. It sounds like I’ve done less fairing than the rest of you would, but the hull continues to improve. What’s another week or even multiple weeks, I won’t see open water til May anyway! :D

Bill

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Re: Polyurethane primer and S3 Quick Fair

Post by fallguy1000 »

Epoxy fairing over primer is fine, but remove any and all sanding dust. I vacuum everything. Sometimes use some denatured alcohol and clean cotton rags before adding more compounds.
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Re: Polyurethane primer and S3 Quick Fair

Post by pee wee »

fallguy1000 wrote: Sun Jan 02, 2022 1:24 pm Epoxy fairing over primer is fine, but remove any and all sanding dust.
A lot of the builders here, if not a majority, use System3 Yacht primer, which is an epoxy based primer. There's no problem going over that with additional fairing, but the OP used a Petit polyurethane primer. Fairing over paint is a bad idea because the adhesion will be poor, fairing over that primer might be a bad idea, and therefore, considering the risk, is a bad idea.

Fallguy's advice about removing all sanding dust is very important. If you can roll the hull outside and hose it down with water it will make removing dust easier, and as an added benefit, while the hull is wet and shiny you can really see imperfections.
Hank

wdr99
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Re: Polyurethane primer and S3 Quick Fair

Post by wdr99 »

Turns out that most of my problems were fairing compound that just needed more sanding. After some 120 grit and a couple coats of epoxy, things look much better.

Now I'm adding some additional epoxy coats to try to get rid of the last stubborn bits of weave that I accidentally exposed from sanding including many areas on the chines. This 12 oz fabric seems to have these heavy threads in it that, when exposed, are a bugger!

Bill

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