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Old 07-18-2018, 01:43 PM   #1
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Epoxy vs Polyurethane

Am about to start picking away at some of the stress cracks and vacant screw holes from over the years on our 33 year old Mainship. Having been mostly a woody guy, I am looking for advice on which resin to use and these "new glass boats". ha. Epoxy or Polyurethane? Repairs will get proper glass layering and Gelcoat finish. Pro's and Cons of each?
Thanks in advance for the advice.
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Old 07-18-2018, 01:53 PM   #2
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I prefer to use polyester resin especially if you are going to finish with gel coat.
Gelcoat adheres best to poly resin paint is best for epoxy. Is what I was always told.
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Old 07-18-2018, 04:06 PM   #3
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You will probably not get a poly gel coat to stick to epoxy. As said above.
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Old 07-18-2018, 07:11 PM   #4
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There are primers and some epoxy resins that polyester will stick to......go to West Systems and check it out...

But the easy, guaranteed.way is start with poly or vinylester and gel the same.
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Old 07-19-2018, 08:55 AM   #5
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IMHO Epoxy resins (and thixotropic additives like poly micro-spheres) are vastly superior to poly resins. I have used them building scale models of boats, RC planes, and some auto repair of damaged body panels. Its not as 'stinky' as poly, dries rapidly and one huge plus to me anyway, is its workable with common woodworking tools like files, sandpaper, etc. I used a West System product. With proper thickener, you can make a very handy epoxy-paste for filling holes, filleting, and fairing surfaces. Now IDK about what top coats are/are not compatible- I did use an oil based enamel paint over mine- and it adhered great. Be forewarned, some people are highly (skin) sensitive to this stuff when uncured........
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Old 07-19-2018, 10:03 AM   #6
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So I turned this up :

https://epoxyworks.com/index.php/app...at-over-epoxy/

Iíve always shied away from gel over epoxy, but the above confirms that the culprit is the amine blush left by many otherwise quality epoxy products. Two solutions, one is to remove the amines on the surface by sanding full cured product and with a water rinse. The other is to use more highly compatible epoxy products, my favorite is always system three silver tip, which I always grab when comparability rears itís ugly head, such as applying varnish over epoxy.

The other thing we should discuss are the better ways to apply gelcoat repairs such that they are invisible and look professional. If you fill a hole and sand it flat you can nearly always see the edge of the repairs standing out as the color differences contrast. Instead, fill the repair just a little shy of the surface. Then sand a larger border around the repair to remove all shiny surfaces, maybe six inches depending on the repair. Finally, spray the visible surface of the repair and blend in to the border you sanded. A preval sprayer is cheap and works well. Then progressively sand the area flat and polish. Do it this way and you wonít see the repair and it has the potential to look as good as new.
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Old 07-19-2018, 10:13 AM   #7
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As mentioned, the amine blush needs to be removed, or use peel ply.
Can't beat West Systems for ease of use. Simply foolproof system. Level of expertise required for excellent repairs is about zero. Watch a YouTube video and go for it.
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Old 07-19-2018, 11:45 AM   #8
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Get used to poly/vinylester and epoxy is just what it is.

Three tools with 3 diffeent characteristics ...yet all have advantages and disadvantages.

The months BoatUS mag has a repair article where Don Casey suggests using poly/vinylester for above waterline repairs.
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Old 07-19-2018, 03:33 PM   #9
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if you have cured poly resin then don't use poly resin as repair use either epoxy or vinyl ester to be sure it actually sticks.
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Old 07-19-2018, 03:51 PM   #10
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Regular poly will work too........many large pro repairs are done with plain old polyester.

Vinyester and epoxy are stronger bonding, but often not necessary.

Often mixing resin types causes stress points and for small stress cracks, sure anything can be used....use what works best for you.

https://www.boatus.com/boattech/case...ign=eline-0718


Using Polyester or Vinylester Resin

For above-the-waterline repairs you can use either polyester or vinylester resin. Of course, for an even stronger repair you can also use epoxy, but not if the surface of the repair will be gelcoat. (You should use epoxy for underwater repairs.)

If you are doing your repair with polyester or vinylester resin, you need laminating resin. Laminating resin does not fully cure while exposed to air, which allows you to get a chemical bond between the multiple laminates you will be applying. To get the final laminate to cure, you simply seal it from the air, either with a plastic or by coating it with polyvinyl alcohol (PVA) mold release.
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Old 07-19-2018, 06:58 PM   #11
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If you are just filling screw holes and stress cracks polyester, vinylester or epoxy resin will work fine. For For larger gel coat areas (larger than a few square inches, I would use polyester resin. To get gel coat to bond to epoxy be sure to remove the amine blush by rinsing thoroughly with water and scrubbing. DO NOT sand epoxy before rinsing since all that does is spread the blush around.


I agree about using laminating resin (wax free) if you use polyester. You can add wax to the final coat of gel coat to get a complete cure. However, remove the wax by wiping the gel coat down with solvent before sanding and buffing. With a gel coat finish you will need to sand it smooth and buff it to a high gloss. Color matching gel coat is also an issue even with white. Unless you are very good at color matching you will be able to see your repairs.
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