Quote Originally Posted by jnordyke View Post
Silicone is silicone...I suspect "marine grade" is a marketing term. I assume these screw holes are occupied - ie have a screw in them? Any of the products mentioned are more than good enough to keep water out. If this is a permanent sealing/no need to ever pull screws back out, then the stainless steel or aluminum JB Weld slathered on the screw threads would be perfect.

Marine grade silicon is much more durable when submerged than the average stuff you get in a tube. Most silicone sealants are not recommend for use where they will be submerged, but the 4200 and 5200, in addition to be much tougher sealants, are recommended for full time immersion.