I'm having trouble sealing two inspection ports in a blue plastic water tank. They were installed by the PO, two 5-6 inch holes, with clear plexiglass plates covering them, fastened with four sheet metal screws, with silicone between the plate and tank. I removed them to clean the inside (pretty slimy) and opted to modify the plates by adding 'donuts' of red rubber gasket material (so I can still see thru the plates to the inside of the tank), and used eight nuts and bolts each, instead of just four screws. Aaargh! When I fill the tank, water still dribbles from both plates. (And not from pressure as the vent line was replaced and the thru-hull is fine.) An archive search mentioned Marine-Tex but that sounds too permanent. I'd still like to be able to remove them. It was nice to be able to get my arm in there and scrub it before using Peggy's shock treatment. Great book by the way. Highly recommended. Anyway, I'm assuming silicone is safe but I've never seen it in black and white. (I'm so anal sometimes.) Is silicone OK, or am I going at this bass-akwards? My first boat, and this is all new.