"David, MM is correct in sayin
"David, MM is correct in saying epoxy is the adhesive to use. I'll generally use Pro-Set or West Systems 6-10 epoxy adhesive. The drawback to these is that they come in two part cartridge form and need a special caulk type gun. West's G-10 should also work fine and can be purchase, I believe, at boating stores such as West Marine and come in smaller bottles. Epoxys are sensitive and must be mixed proportionally correct and thoroughly blended. Grind away ALL the old adhesive and roughen the fiberglass surface with 80 grit, clean well with acetone before and after sanding. Of course make sure the puck is set to shoot straigth down and not angled. You don't mention if this is a "shoot thru the hull" sender or an external surface mount. If it's an external, clean as mentioned (remove any & all bottom paint if used) & use 3M 5200 sealant. Temp. secure the sender in place ( blue mask, duct tape...) Give it enough time to set up and cure (you can use a finer grit paper on the gelcoat w/ 5200). If it's a "shoot thru", cutting a paper cup or a plastic mixing cup, cut to the same angle as the dead rise and taping it in place will help (cut the bottom out also). Pour in the adhesive then set the puck in place. Give it at least 12 to 24 hours to cure. That should do it. I would not recommend using JB weld for this type of install or any automotive epoxy adhesive/filler that might} contain any metallic type fillers.
If past attempts failed it's no doubt an adhesion problem & may not be specifically related to the products used."
