I was told that silicone is bad for grp as it will "eat" it apparently. I can't tell you that it's true, however since then I've gone for Teflon instead. The car paint won't absorb the silicone whereas GRP might.
Silicon will damage your gel coat horribly cos it's rock.
Silicone on the other hand is only a problem cos you can't get it orf agin. Bit of a problem if you need to do a repair.