Not intending to shoot you down, but this does need correction. Please feel free to list your specific experience.
H2Oaltitude wrote: ↑Tue Dec 19, 2017 5:17 am
Plastic bag will bond to epoxy and epoxy will yellow w/sun.
I work in the Marine industry doing fiberglass repairs specifically with West System 105 and G-flex. Both epoxies DO NOT stick to LDPE/HDPE (zipock and almost all other no-name brands) bags or any other polyethylene plastic. Actually, nothing does. If you come up with something that does bond to Polyethylene, you are rich enough to never have to work again in your life. If you look up the industrial processes and few chemical treatments for doing this, you will understand how difficult this plastic is to bond to. My only idea of why you may have experience epoxy sticking to new plastic bags is if you had some folds in the covering piece of plastic. This would "lock" the plastic in to the repair. I Even use "Saran wrap" as a smoothing cover to eliminate some sanding in the finished repair/coating.
AND
UV is a huge issue with West System 105. When used as a coating on wood, you have to put a highly UV resistant polyurethane coating on top of it to protect it or you have to paint it. This is the reason why West System 105 is extremely unpopular for surfboard construction - no part of the surfboard could be clear without another (more weight) coat of UV protectant. However, for smaller repairs where maximum structural integrity is not required, you can just let the West System 105 yellow and become brittle. And given that a surfboard is not in the sun all the time like a sailboat is, you really don't need to concern yourself with this too much in a TT or surfboard repair unless you are going to leave it in the sun for a month straight.
For low heat build up instances like the OP's repair, waxed paper (next to the aluminum foil and saran wrap in the grocery store) is the way to go. It is pretty stiff for keeping contours (don't use on compound contours). Only when extreme heat is applied, is wax paper a no-go. So if your pour is going to get hot (about 3x3x3 cube of epoxy in your repair), the wax will melt and the epoxy will infiltrate the paper. This is not a big deal if you are sanding anyway, but wax paper will not leave a finished surface if heated too much as some of the paper will remain behind. Also make sure that if you are using external heat to make the epoxy set faster, you will likely not get a good finished surface with waxed paper.
