g'mornin Graig,
Butyl doesn't adhese too good. It is a rubber bedding material classed as a sealant. It's major contribution to the caulking wars is that it remains flexible, which is where waterproofing comes in. The 'sheet' stuff or sticky tapes you get from auto stores are recomended by New Found Metals, eg, for mounting the inner parts of their ports. The stuff is black so they run a cosmetic bead of something white around the squeezeout areas (Sikaflex 291). S291 is a fast set polyurethane.
I think it is agreed that polyurethanes and even polysulfides loose flexibility over time from UV and boat lotions. So here is where your super adhesive gets leaky. I don't know that I would use tube butyl (comes in colors and white) to bed rails on the cabin. I might try the tapes if they came in white, because you want a 'caulk line' On the rails I wouldn't want to squeeze out any caulk or adhesive I put there.
Might try the approved stanchion base method on the rails which is to spread an even thick layer (GOOD LUCK!) of caulk, mount in place and then semi-cinch the fastenings so that everything is even and positiuoned, let it harden to some degree (some will say partially - others will say all the way) then drive the screrws home, hopefully pulling the rails down so that the caulking bulges. (Sheet or sticky gasket material would be SO much easier.) You will have done some carefull blue taping. Last resort would be to cut the bulge off when you leave the tape on too long. Could look real smart!
If you decide to use tube butyl to mount the rails because of its reputation to always flex, here is a trick I just read about. That is to use O-rings around the fastening holes to avoid squeezing all the sealant out. For that matter, if I had some of the tape which is much more firm than butyl out of the tube, might try doughnuts of that around the screw holes. And finish up with tube butyl or polysulfide.
Might go with 4200 - but polysulfide has a rep for lasting longer exposed, and renewing is 100 times easier.
There are issues mounting the hand rails to the cabin. My guess is that witout elaborate oversized drilling and filling the cabin/liner the rails will always want to move. Injecting epoxygel inside the cabin/liner can't be taken lightly, as that can be even more permanent than 5200! The liner is dimpled and distorted by the screws so you have to decide what level of restoration you want.
For a quick fix I would use polysulfide.
I always think in terms of having to take things apart later, curse of the maintenance man. I think dissimilar materials should be bedded, but that's just me - somebody else wants to glue them together, that's ok too! But I'ld have to be starving to take the job of fixing the leak that is bound to show up.
NFM recommends using a metal spatula heated with a hot air gun to soften 5200 when you're trying to remove it!
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Butyl - which is so cheap - might also be considered to bed the coamings. It would be great if an inexpensive material could be useful on the boat. Cleanup is with not so bloody lethal mineral spirits. Hope someone trys it here and reports.
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