Originally Posted by HopCar
Actually I was just kidding a little. In this application where you are mounting metal to fiberglass I would use 5200. It is very easy to break the bond between metal and 5300 by warming the metal. You are then dealing with the exposed 5200 which can be attacked with a razor blade and a solvent like DeBond.
All of my deck hardware and thru-hulls were bedded with 5200 and never leaked in almost thirty years.
I inject epoxy into holes and it works pretty good.
I use 5200 for things I don't want to take apart - thruhulls, stanchion bases, cleats, winch bases and all sorts of stuff. BUT, I'm really careful about how to do it.
I dry fit what i am mounting / installing. I tape off around what I'm putting in, lets say a stanchion base. Usually I also tape off the edges of the base. Then I remove the base and drill a small countersink for each mounting hole then squirt some epoxy in the holes. The last prep step is to put down a layer of 5200 over all of it.
After all this I install the stanchion base. I tighten the screws as much as I safely can and most of the 5200 gets squeezed out. When tightening the screws in I add a little 5200 just under the head of the screw. Once it is all tightened down I wipe off everything as much as I can. Once it is all clean the last thing I do is pull up the tape while everything is still wet.
All of that leaves a nice, tight, usually waterproof install. The countersink in effect makes a small washer / gasket of 5200 under the stanchion base.
I forgot a step! I clean everything with acetone before the epoxy and 5200.
... you only wanted to know how to stop the leaks and all the screws still feel tight...
Back out the screws 1/3 to 1/2 way. Force 5200 around the the screws and fill the threads. Tighten it in back in and make sure there is enough 5200 to squeeze out around the top of the screw head. Then wipe it all off and forget it till the next rainy season.
I also use new driver bits and sockets / extensions and either a ratchet or a shoulder brace to drive in big screws.
As I've gone this far on my rant I'll add that I have a handheld impact driver and a propane torch to get tough ones loose.
But yeah, 'to do the job right' you should put in a backing plate and use bolts and still tape it all off etc. But if everything still feels tight just back out those screws and screw them back in with the 5200.