Ok, here is the work I have done on painting (neglecting any tank sealing or body modifications).
1) Strip paint off of tank using sandblaster or chemical stripper.
2) Sand the tank with 80 grit and make sure rust is removed.
3) Buy quart of bondo and use to fill low spots according to directions on can.
4) Sand with 200 grit till smooth, use glazing putty to fill in any small dings and resand.
5) Wipe down with Isopropyl Alcohol (rubbing alcohol with 70% or higher).
6) Wipe down with tack cloth
7) I sprayed on a primer (with hardener) using a rather inexpensive air gun and then had to sand because I used too high of pressure with a siphon feed gun. Went back, thinned the primmer and dropped my air pressure in half and it did much better.
8) I then wet sanded down the primer. Due to my early spray method I ended up with what looked like lots of pin holes that would take me almost to metal to get them out. So I filled them with glazing putty and resanded with 400 and 800 grit paper (see image of sanded tank attached).
9) Wipe down with Alcohol again and wipe down with tack rag to pick up any dust or lint.
10) Mix paint and spray.
11) Mix clear coat with hardener and spray.
12) Wait a while (week or more) and then finish sand clear coats and polish to remove any sanding marks.
Sorry for the abbreviated version, there is more to it of course, but that is the general outline. The main thing is don't get impatient and put lots of work into the bondo, sanding, and the primer and sanding. Use a rubber block to hand sand as this prevents finger grooves. Remember to clean the paper as a buildup on the sandpaper will create grooves in what you are sanding. Also, sand from multiple directions to get the body work flowing in all directions.
Lastly, on the underside I coated the primer with a heat resistant paint prior to the color coat so as to not have to tape off the tank when I went to apply this coat later.
To paint I hang the tank from the rafters in the garage in my paint booth (plastic sheets on all 6 sides) and then use one hand to steady or position the tank while spraying. I found that a finer mist works best as it only needs enough that it doesn't flash before adhering to the surface and then I don't get any runs. I just get to repaint because my cheap gun resulted in some spittle on the tank. I am going to wet sand it down, put on another color coat or two to get something for the clear coat to adhere to.
The lesson learned is to buy a better gun. There are some decent devilbliss guns on e_bay that are worlds better than mine. A gravity feed HVLP gun would be best as you could then spray more than just a base coat. You may want different sized nozzles, mine only has a 1.4mm. After the gun the compressor needs to be big enough to keep the gun going so the paint doesn't setup while waiting for more air.
Anyways, here is the tank after priming, sanding, filling, and sanding again (though it wasn't washed yet). Feel free to ask if you have specific questions, however bear in mind that I am learning this as I go and I may not be the best person to ask.