epoxy or polyester resin?

[shadybacon]

Hi Everyone,
I just purchased my first daysailer a few weeks ago and I am replacing the stringers (among other things) before I put her in the water.
I have been following the thread on stringer replacement with great interest.
My question for the group is this: Are the epoxy resins (West System) superior to regular polyester resins for repair jobs? And, if so, are they so much better as to justify the much higher cost? And, lastly, can anyone tell me why they are better (if they really are).
Any help will be greatly appreciated as I am making this repair this weekend.
Thanks
Jon

[GreenLake]

Jon,

Epoxies are stronger, and they are said to bond better to cured polyester than polyester resins. Of those two factors, the bonding strength is the more interesting one, the overall strength depends on the dimension of your laminate, and is not critical for this application.

Another advantage is that epoxies don't "stink" as much - the styrene catalyst for the polyester is rather nasty stuff. (With epoxy you're fine on fumes, but definitely want to avoid skin contact).

Epoxies require very precise measuring so that the two compoents are used with their exact ratio. For polyester, the amount of hardener can be varied and affects how fast the resin cures, not so much whether it cures. (Mis-mixed or under-mixed epoxy will not fully cure).

There are some additional differences in how to work with both - so if you are very familar with either one it might make sense to stick with that type of resin, or, if you want to switch to something new, to really read up on the application directions. West Systems has a great publication that describes stringer repair with epoxies (applies to epoxies from competing firms as well).

Epoxy, but not polyester, is able to fully moisture seal wood, by penetrating and then hardening partially inside the wood. If you plan on using wood for your stringers, that would be a consideration.

Gelcoat will stick to polyester, but epoxy may require a 'tie coat'. SystemThree markets one that should allow Gelcoat to be used on epoxy. For your stringer repair, that's not an issue, but if you ever repair your hull and wanted to patch the gelcoat over an epoxy repair.

Epoxy is not stable under UV - simply means it must be painted or varnished. (Clean epoxy with soap and water to remove "amine blush" prior to painting and use primers where required).

Not a complete list, but I hope I've managed to hit most of the main issues that differentiate the two. My personal advice: for repairs, use epoxy.

(For links to publications on the web, see the other post).

[K.C. Walker]

What Greenlake said!

Jon, welcome to the world of the DaySailers!

The bond strength of either is probably adequate for this job. West System says with a polyester repair the bond strength can be about 70% of a primary bond, as with the epoxy it can be 80% as strong bond. http://www.epoxyworks.com/22/fiberglass.html

KC

[shadybacon]

Thanks for the info. That was exactly what I needed.
I think I am going to go with polyester resin to save money.
Next I start on making new coamings.