Just my opinion......don't waste your time. I had a cargo truck that started leaking around the screws. I cleaned around the screw heads very well and applied the silicone. It's VERY time consuming. Well, it fixed the leaks for about 3 or 4 months before the silicone started separating and the leaks returned. I ended up replacing the screws in a fraction of the time it took to try sealing them.
The silicone would probably work ok if you backed out each screw enough to get the silicone under the head, but if you're gonna do that, you might as well replace them.