I would use an MMA one with a longer open time. While the MMA will bond very well to the PVC & gelcoat your bond will only be as strong as the Gelcoat is to the fiberglass. With MMAs sanding and prep is not necessary for FRP to FRP or Metal to FRP or Metal to metal bonding. Generally bonding to gelcoat is not a problem. In your case, yu are looking for a structural bond.If there is a failure, it will most likely be the gelcoat debonding from the fiberglass. My recommendation would be to trace the rubrail and sand off the gelcoat so that you are bonding to a stronger substrate (fiberglass). You should not have adhesive failure as most quality MMAs have 3500-4500 psi tensile, which is the style/motion of force on the spray rails. Fiber tear will occur at 1500 to 1700 PSI on most typical FRP. A few application tips; don't bond if you exceed the open time, work time, pot life of the material as it will not have enough "power" or time to resolvate the cured FRP. Once you have the rail clamped or fixtured, do not move, as you may disturb the chemical crosslinking and the chemical bond that is occuring. Use some fishing line as a shim, so when you do clamp you Do NOT squeeze out all or most of the adhesive. Most materials test data is at 74 & 72 deg F.. Pay special attention, if a material has a 10 minute work time at 74f and its 94 f your work time will be cut in half, and vice versa if temp is 20 degrees cooler.
I want to take the photos also, I may be in Stuart for the Martin County nautical flea market.
BTW I do have a couple photos of my buddies 19 Key west next to it.
