Have you got access to the plate at all?
I have redone many of my boat rail stanchions over the years. Rewelding, TIG, the joints between the tubes and the foot plate and grinding the plates to eliminate the sharp corners and edges.
That required me to deal with a LACK of backing plates although the feet were through bolted with 316SS.
There was NO access in many cases to the underside without destroying parts of the interior.
What I made was aluminum backing plates, each hole fitted with Helicoils of the appropriate size and pattern to match the stanchion feet..
I also installed one much smaller heli coil, 6-32, centered so I could then use a 6-32 threaded rod to lift the backing plate into place. I would drop the rod through and then spin the plate onto the end so the plate could be lifted into place.
I made a pattern of another plate, tapped holes, with the same size of securing bolts installed that the stanchion feet used for centering. I drilled a 5th hole centered for the drilling of the 6-32 lifting rod hole.
I loosely installed two of the larger bolts temporarily into catty corner holes to hold the backer in place while I then secured the 6-32 screw into place.
Once the 6-32 screw was holding the backer I then removed the catty corner bolts and finished the install with my 316SS bolts with appropriate sealing/caulking.
It was still a bunch of work of course but it enabled me to provide a backer plate and reseal the stanchion plates.
My only regret was this was mostly done long before I became aware of the Butyl rubber tape however they still don't leak.
The aluminum backer plates were 3/8" stock.
As I went I also Never R Seezed the SS bolts as they were installed so I would not be drilling them out if I ever needed to go back.
