I have a lifetime in automotive, aircraft, and marine application refinishing.
Here's my suggestion.
If the tank is brand new aluminum (brushed finish) then I can almost guarantee that it has a clear coat on it to keep that metallic luster, otherwise it would get dull and form a patina.
You'll need to strip off that clear coat with some paint stripper or sand it off with 80 - 120 grit on a DA sander (dual action sander), an electric palm sander will do.
After you get down to bare aluminum wash the tank first with alumaprep 33 (phosphoric acid solution) it will clean the aluminum down to the pores. Follow the directions on the bottle exactly.
Next, brush on a coat of Alodine (chromate conversion coating). This will make the aluminum surface receptive to any topcoat starting with primer and is the best insurance against future delaminiation problems or peeling. Again, follow the directions exactly.
After all that's done, spray a coat of Sem (brand name) self etching primer on the tank. A couple thin coats will do. This is available in a rattle can.
Next, do your artwork. The only problem with using artist oils is that they are linseed oil based and it will be difficult to clear over the artwork. You're better off using acrylics or getting online and getting some Createx Auto Air, it's an automotive airbrush color that's water based and can be clear coated with any urethane automotive clear. The other option would be to use One Shot sign painters enamel. That's an air drying oil based outdoor enamel similar to rustoleum but much better. The only problem with One Shot however is that it will be difficult to clear over it with urethane becase it could lift. To avoid this problem you would have to drop a little urethane hardener in your One Shot colors so they will cross link cure and not just polymerize with oxygen. Doing this then gives them a "pot life" which means you only have so much time to work with them before they chemically harden.
One thing nice about one shot is that it doesn't need to be cleared, it's designed to be a final coating for exterior durability.
All the above materials will be available at an autobody supply store. You will not find any of this stuff at Home Depot.
After all your artwork is on I would take it to a body shop and have them hang it in their booth and blast some clear on it.
All this could be done cold and there would be no danger of compromising the integrity of the tank. I have done this many times back when I was putting flame jobs on tanks.
The other thing you might want to do is drain the air out first and remove the valve. Go buy a 3/4" bronze square ended plug and put that in. I drilled a hole sideways through the square nub on the plug so I could run a wire through it to hang it.
It's going to cost way more than the tank is worth but I'm sure that doesn't matter. Whenever we take on these projects it's not about finances, it's about cool,.. right?
