"Can I put 5200 right on the grinded edge?"
yes, of course - AFTER the patch is complete and securely
fastened into place, primed, then the sealant goes on.
3M-5200 is not structural - it is an adhesive caulk used for waterproofing.
there are MANY very talented metal workers here on this forum !!!!
some, like myself, would not hesitate to mix/match materials to get the
job done. If a large gaping hole is on the top, I would either use
steel skin, aluminum or even 1/8" PVC sheet just to keep the rain water out.
on the bottom, of course, it has to be more structurally sound and watertight.
also, IMO, there is no such thing as "non fixable" metal...... I have worked on too many
car and truck bodies to know that. It is like surgury - cut back all the dead tissue
and apply clean grafts and dress the wound - just that simple.
if this task is not passing the test of your welders, maybe it is time to switch to aluminum floats.
The gallery can only give advice after knowing all the facts.
such as - is the big hole on the top or bottom
photos of how it was mounted to the frame
what are your expectations after the patch
what kind of waters are you in ?? lake, river or white water rafting.
rocks or sand - is it in the water all the time ????
stuff like that, can offer you more accurate feedback.
personally, I have learned to prep the metal and prime before any kind of sealant.