it had stayed on for 3 years but now won't. the circular area the suction cup held on to had tiny raised spots where the paint seemed to have been pulled into sharp tension points about 0.5 mm high. i scrubbed the area down with a 3M abrasive pad until smooth. then it held on only a few hours.

is there a substance one could apply that would make the suction cup work as when new? perhaps silicone grease?

any ideas?

why not get rid of the suction cup and hard mount it with screws or epoxy
if not go buy a new suction cup

"paint seemed to have been pulled into sharp tension points about 0.5 mm high"

That sounds like osmotic blistering caused by the effects of dampness trapped between your suction cup and the gelcoat. Your gelcoat is permiable. If you have abraded the surface to remove the "raised spots", you have taken the extra step to promote even more moisture problems to continue the spread of "cancer". Once moisture wicks into the raw FRP (glass fiber)it will seep behind the gel coat and begin to create more of those blisters you have "scrubbed" down. Water is stealthy and always causes damage. There are much better methods to secure a transducer than screw holes and caulk...and obviously suction cups! It might be a good idea to correct the problem before it becomes any worse.

sorry about neglecting to advise the boat is an aluminum jon boat (alumacraft) with the typical olive drab paint.

Buy a peice of lexan and glue it where you want to mount the transducer then sick the suction cup to it

Well, I suppose that pretty well changes the complexion of things so far as suction cups go.

is this through the hull? if so, use some duct seal to glue it down.