bertram31.com

CaptPatrick, I going to start doing some repairs inside my B31 and the question I like to ask is : should I use polyester resin or epoxy resin ? I really don't know what is the difference except on price (epoxy is expens.)and here, many people have their own opinions but I keep lights off on this.
I go to made some changes on engine rooms and a major refit on the inside like I sawn here on some posts. Also go to exchange engines and for that now is the time for the one and only big repair at least on my life time.

Thanks for all you can do to enlight me on this matter

Luis Ramos

Correct me if I'm wrong:

Epoxy resin is stronger, more "water proof" (resistant), and lighter.

Downsides: more expensive, future repairs to epoxy resin will also have to be done with epoxy resin.

Epoxy repairs will bond to polyester resin, but polyester resin will not bond to epoxy resin.

Do I have this right?
-Joe

Oh, and you can't apply gelcote to epoxy jobs since gelcote is polyester based. That's my understanding anyway.

Joe, you're basically right...

Luis,

The decission to use epoxy, (a true good adhesive), or polyester, (relatively poor adhesivion), can be pretty much divided into two groups.

Group One consists of, (but not limited to), structural components to be added to the boat, repairs or modifications that are below the water line, items which wood is part of the structure, & items that won't be subjected to heat above 200ºF.

Group Two consists of those items that don't fall into Group One...

G1 gets epoxy, G2 can get away with using polyester.

Any polyester resin that is used over cured & aged fibergass, (that was originally done in polyester/vinylester resin), will only have a mechanical bond and must be properly done to acheive a good execution. Old surfaces must be ground back by a minimum of 12:1, (if the substraight is 1/2" thick, the grinding must extend 6" in all directions...), the ground surface must be clean, free of dust, smooth, and primed with a complete, but very thin coat of catalysed polyester resin. Immediately after priming, use a paper towel to wipe away any excess resin. You want the resin to completely penetrate the pores of the ground surface, but not to have any depth above the surface...

The first layer fiberglass should be no heavier than 3/4 oz chopped strand mat. This layer is called the skin coat. I prefer a lighter material called veil for my skin coat. The next layer will be 1 1/2 oz mat followed by what ever you've decided your layup schedule to be. For non structural work, mat is fine. For stronger & more structural items, 12 or 14 oz biaxle cloth can be used. Each two layers of cloth must be seperated with a layer of mat.

The above techniques for polyester are the same for epoxy, but because of it's classification as a true adhesive, anything done in epoxy will, by the nature of it's bond strength, be stronger and more moisture resistant.

On my own projects dealing with a fully cured substraight, I find that I use polyester resin about 60% - 70% of time. The rest of the time, or when I'm in doubt, I use epoxy.

Epoxy: Longer to cure, more shrinkage than polyester/vinylester, harder to sand than polyester/vinylester, more expensive than polyestyer/vinylester, low heat tolerance, & doesn't disolve the binder used to hold chopped strand mat, therefore a slightly lower penetration of the mat. BUT IT STICKS!

Under certain conditions, polyester, (including gelcoat), can be made to stick to epoxy, but it requires that the epoxy surface be heavily abraded & primed with a special polyester primer called Duratec, (do a search for Duratec gray surface primer Hawkeye Industries). But as a general rule, Joe is right: Epoxy will stick to a prepared surface of polyester, but polyester will have a poor bond to an epoxy substrate.

I've never paid much attention to the density differences between rpoxy & polyester/vinylester resin, so can really comment there... Vinylester was formulated to be an ester based resin that has more of the bonding strength of epoxy, but it's more like polyester than it's like epoxy.

Br,

Patrick

Again you make things clear and for that I only have to say THANK YOU. Also to Face that quickly reply to my questions. Now, I think I have a better idea on this topic and I can start doing some training before I go on the real work. Thanks again
Sometimes people ask me why I don't buy another boat and leave my old Bertram 31 - I don't answer just smile because they really don't know what they are talking about.
Luis Ramos

Tell them it is like fine wine . . . !