I have a good start on making some auminum flanges for an aluminum gas tank. The basics are, one flange inside and one outside with the filler plate/neck adapted to the outer flange with studs and nuts. The inside and outside flanges will be held together with a separate group of FH stainless allen 1/4-28's. I want to seal the thread area where the FH allens go through the outer flange, then the tank's upper surface and into the inner threaded flange's threaded (12) holes. The flanges proper will have a suitable gasket. Seems like there ought to be an RTV that will do the job. Recommendations?