Foolproof,
Sorry if this is a bit lengthly. I too am in the middle of a large project on my 1980 boat. It started with deck sag under the mast and ended up a whole topsides redo.
I decided that after seeing within a year my deck gelcoat (or what was under it) started to disappear in spots. I found a paint manufacturer online which apparently went commercial called Supermarine. They state that most of their works were government vessels. In prepping to paint I removed all the thru deck fittings from the deck including chainplates & stanchions, everything. I found various caulkings used over the years from the original RTV thru 5200, thru 20 year latex.
To my surprise, here is what I found. RTV I believe no one uses in boats anymoreas over time it’s like hardened Liquid Nails. 5200 is a bitch and should only be used below the waterline or in a once only compression application. If the sun bears on 5200 and you have movement such as stanchions etc. you will notice cracking on the exposed areas. I have to assume that the crack will lengthen, as 5200 is not pliable over time. Not to mention having to rebed a fitting caulked with it before. The best way to use 5200 is if you only have to do it ONCE.
When I got around to fittings bedded with silicone caulk here is what I found. I used the clear silicon early on around 1990. After some years when the 20-year latex caulk came out, I thought this has to be better/easier for re-bedding and cleaning off the old caulk. So I used this for items I would need to redo every couple of years (stanchions, chainplates etc.)
When removing the latex bedded fittings I noticed that it did not even out for a total seal like I assumed it would and these were fittings I had leak problems with. Therefore the seal did not hold up.
But, all of the compression type fittings I removed that had silicone had a full seal throughout. The unexposed caulk was still pliable. and easily removed with acetone & scrubby pad. None of these fittings ever leaked. Man was I surprised.
I picked up some DOW contractor grade silicone caulk and will rebed my compression type fittings with this. I put in a new pane in my 9x9 hatch. The plastics guy sent me to get the DOW caulk, as the original seal was 30 years old and still pliable & waterproof. Silicon just like another caulk will over time degrade on exposed edging from UV, saltwater. temperature, age. But this DOW stuff still looked in great shape.
My sail tracks are concave underneath. This makes sealing a breeze since there is room underneath for ample caulk and completely out of sunlight. I would suggest a matching backing plate underneath for evenly spreading out the load.
Just a thought, an old salt told me when rebedding, apply caulk anywhere up to 1/8" approx. thickness. The secret is to only tighten the fitting until an equal amount of caulk squeezes out on all sides (.060"-.090"). Then walk away for 2 days. This allows time for the caulk to setup. Come back and tighten as necessary, trimming off the excess. And you now have a complete seal throughout. Never tighten all the way, when you caulk as all the good stuff is squeezed out & wasted. And your "gasket seal" is paper-thin.
Another thing I try to do is never let the topside screw head/bolt turn in the fitting while tightening from below as this can cause you to lose caulk seal between the screw/fitting and thru hole.
Sealing the upper shroud chainplate is a pain between the vertical chainplate and the cover plate (if that’s your setup circa 1980 thru 1984 I believe). I think the DOW caulk will hold up with better adhesion for sealing. Black caulk holds up better/longer in UV conditions.
So I may use latex for small thrudeck fittings, silicon for larger fittings (especially tracks, chainplates, stanchions etc.) 5200 only for one time stuff.
Hope this helps………..
CR
