G'Day Mate, well asking how "thick" a coat of paint is, is akin to "How long is a piece of string", ha ha!
The main thing to remember is that the only reason that you're sanding the tank at all is to "flatten" the primer/paint for either another coat, or in the case of clear top coat, (if used) to prepare for buffing.
The beauty of spray cans is that they spray a very fine coat compared to the volume of paint you would apply in one coat from a HVLP or high pressure spray gun. The bonus here, is that the finer you coat it, the less opportunity you'll have for runs and sags, but conversely, the easier it is to rub through.
My advice, for what it's worth, is to build each layer with several coats allowing 20 minutes or so @ 60 degrees F, between coats. Allow each layer (etch primer, primer, color coat, clear coat etc) to dry for a minimum of 6 hours, or preferably, overnight.
Only rub back enough to flatten each layer, and if you do happen to rub through to the substrate, touch it up with the appropriate finish, and allow it to dry accordingly.
As you can see from my pics, rattle cans certainly have their place in our world, but like most things, patience, and attention to detail are two very important personal qualities that will determine the finished product.
I have neither quality, but over the many years, and many disasters that I have been responsible for, I have taught myself to be patient, and not to cut corners, at least when it comes to painting. Have fun! Cheers, Terry.
