Bender of Nails
Join Date: Jan 2009
Location: West Coast BC
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Brian and sailingfool are bang on.
Mounting the stanchions as you show will trash the gel all around that radius and will let water in eventually, regardless of how well-bedded it is. For a good fit, mix epoxy and cabosil (fibers). Tape the stanchion base with packing tape so it doesn't stick and bed it the way you would with sealant. You can add some talc or white flour as well so it'll be easier to match the paint to the gel later.
Aluminum is a WAY better for a backer in your application. 1/4" will be plenty rigid and easier to work than your 1/8" steel. If you have some other reason for using steel (like if the backer were outside), grab a 5" angle grinder (about 40 bucks) and some cutoff wheels. They make wheels specifically for stainless and they're just a few bucks each.
You can cut aluminum with a normal jigsaw. Use a wood blade so it doesn't pack up and wax it with a candle (or Alu-Cut). The chips are annoyingly hot.
"Verbosity leads to unclear inarticulate things"