What silicone brand?
Some will tell you that you use a marine grade silicone, but here's a money saving tip.
All marine grade silicone is is 100% silicone. You can get that at Home Depot a lot cheaper than you can buy marine grade for. GE make Silicone I and II. The difference being that one of them has a mildewcide in it (can't remember which one does).