Hello, A couple of questions...

The 3-m Seam sealer is beige in color (other colors may be available.) approx $7.00-$9.00 per tube.
You use a simple caulk gun from Home depot is what is needed another $5-$6.00 for a cheap caulking gun.
The 3-m seam sealer (my favorite) dries to a skin in like 30mins and you can paint over it at that point.
When it gets hot it does not loosen up like the tar suggestion.

After the bare metal (sorry never used acids on the panels in my life- partial to sand blasting... draw back to blasting is sand gets everywhere!) I use a two part etching primer. Rust Oleum has an automotive product in the aerosol can, same color army green, but takes at least 24hrs to dry before next layer or primer or paint. I've used it for years and only down side is if you paint too early the paint will peel over time. If you wait the full 24hrs to dry... you have to lightly scuff it up with scotch bright and be careful not to break thru the green to bare metal. If you do, you need to spray green again and wait another 24hrs.
The body shop version ( a true two part epoxy mixture) may not be available in all states unless you are a licensed body shop. In Connecticut with my business license I am able to purchase professional stuff like that but I think any one can here. Nice thing about the body shop professional version is if you let it flash the 45 mins you can coat it directly with the next coat with out sanding it.

As for the top coat, rust oleum is used by a lot of places for inside floors because it wears well, goes on thick (sound deadener) and when dry is able to put the sound deadener pads right on top. And it is not a visual because its under the rug.
Good luck on your quest for the correct method. Don't rush it because skipping a step or bad info will bite you in the butt every time.
Joe