The stuff I was thinking of is similar to what has been posted here, but I got mine from my mechanic who is a NAPA mechanic. It is a black spray can that says "Paintable Rubberized Undercoating." After I take care of the rust on my front bumper and prime, I was going to spray this stuff on before spray painting
with a Rustoleum bumper paint
. Ran out of time this summer but in the plans for next year. It should help in your situation too; often called "chip guard" as it protects whatever its sprayed on, kind of like a rubbery coating that should deflect road debris for you and keep that area more or less waterproofed, which is what you're looking for.
By the way, my Scamp
had been in an accident with the PO and when I took to a reputable gelcoat boat repair guy here he told me that gelcoat is not waterproof. I never went any further with the conversation, and not like me either as I always want to know about how everything works. Must be porous to some degree. Anyway I've seen his work and it was top notch, but alas, I couldn't afford it at the time. Maybe some day!
Best of luck with your project,