Thanks for your thoughts GL.
In my research last week I turned up this 2012 post:
The Hull Truth - Boating and Fishing Forum > BOATING FORUMS > Boating How-To’s : The Kerno Memorial Forum >
What sealant to use around screws?https://www.thehulltruth.com/boating-how-s-kerno-memorial-forum/466901-what-sealant-use-around-screws.htmlwhich referred to this excellent, illustrated step-by-step 2003 article about butyl tape by someone in the business, 'MaineSail' (a "Super Moderator") on SailNet:
Bedding Deck Hardware With Butyl Tapehttps://www.sailnet.com/threads/bedding-deck-hardware-with-butyl-tape.63554/in which he addresses when he does
not use butyl tape:
Stuff under the water line, through-hulls near the water line, and anything that may be exposed to significant amounts of fuel, like the diesel deck fill, should also not be bedded using Butyl, since butyl dissolves in petroleum products.
[...]
While you probably could use butyl below the water line I don't, and use marine sealants instead such as Sikaflex 291 or a Polysulfide instead. 3M-101 is no longer marketed so you are left with Boat Life Life-Calk as the sole marine grade polysulfide. I never liked Life-Calk as much as 3M 101 but it's still better than "Satan's Glue", AKA 3M 5200.
#5 Tube Caulks - When you need to use a tube caulk for deck fittings choose one with LOW ADHESION. Polysulfides like Life-Calk are generally the lowest in adhesion but Sikaflex 291 @ 260 PSI and 295 UV @ 160 PSI are both less than 3M 4200 @ 300 PSI or 3M UV 4000 @ 300 PSI and a better choice IMHO. I generally prefer Sikaflex 295 UV for deck stuff and Sika 291 for below water, both 295 UV and 291 are polyurethanes with a 500% elongation rating.
It was this butyl tape article that led to my choice of Sikaflex 291, which I ended up ordering.
Sikaflex®-291 is a 1-component, marine grade polyurethane elastomeric adhesive and sealant. Used by many boat builders, its fast cure time makes it ideal for applications where speed is important.
https://usa.sika.com/en/industry/global-industry-content-pages-to-keep/commercial-vessels/elastic-sealing-bonding/watertight-sealingbonding/sikaflex-291.htmlHaving now removed the centerboard, whose setup is identical to Rakozy's 2015 photo, I found that the stainless plates were bedded with a white compound that is still soft and pretty sticky too. The bolts (not screws) appear not to have anything on them ... save perhaps a little of whatever was used to bed the plates. I hadn't actually thought of putting anything under the plates. . . . (Regarding exposure to fuel . . . although I don't have a motor on my boat, it is moored for 2–3 months in an area highly trafficked by motor boats.)
It was great having Rakozy's photo. Since the CB set up is functional, I decided to leave the centerboard resting on the inverted hull with the cable attached while I put 5–6 inches of antifouling paint in the CB trunk and the leading edge of the very top of the CB. Then replace the CB and paint the leading edge while vertical. Lastly paint the rest of the bottom, but that's another thread I'll read up on. . . .