I really like the Marson Platinum filler too. I also use Evercoat EZ Sand for a glazing putty to fill in small pin holes, scratches, etc as needed.
The epoxy can sit indefinitely, but after 7 days needs to be lightly sanded if you want to apply anything to it. If recoating or applying within the 7 day period then sanding is not necessary.
Don't exceed 1/4" of filler, and stripping to metal is always my preferred choice but is a lot more work as well. It depends on how you want your final product. If you're going over an area that doesn't have any filler on the original finish, you could do some light sanding then epoxy without going down to bare metal.
I used to use those butt welding metal clamps too. Did them on my quarter skin seam, but we all know how that turned out...