I found that out by mistake. I figured that if I filled a ziplog bag with water / gel mix, I could use the weight of the bag to compress the fibreglass to conform to the sharp edge of the stern bottom. Well I used a vinyl bag I got from my lab. To my horror, the bag stuck like crazy. I have to spend 2 days to slowly scrap and sand the bag out because it have blue color words print all over it. I assume that it may work if I lined the bag with a trash bag.
Well, we all learn from our (or preferably someone else's if you read ths BBS) mistake.

: One of the things I had to do when trying to recuperate my boat from the
: "skin cancer" (uncured opaque epoxy) was to sand it to the bare
: wood.

: Prior to the epoxy mess I had applied a self adhesive vinyl banner (a banner
: cut from a vinyl sheet with a some special plotter at a design factory) on
: to the wood.

: It worked great, and beneath the fiberglass looked like a painted banner.

: When I sanded the whole thing out, I had to work a LOT to peel this thing off
: the boat. The adhesive of the vinyl plus the resin applied over it
: actually stick it to the boat like a tatoo.

: Hope this helps

: Marcelo