I don't know if you understand just how poor a quality nearly all rattle-can paints are but almost any of it will wipe right off with a rag soaked in Acetone. Of course that will only take you down to the original paint, and there you are going to have to sand.
As for the tank, you have two choices and if you had posted some good photographs of your parts I could probably save you to the trouble of having to make the decision yourself as to which way to go. But you didn't post photos. So, here are your choices, you either have to take it to bare metal, or you can leave the original finish on there if it is sound and just prep it for the next paint. So that's the key, is your original paint sound? I don't mean has it faded and I don't mean does it have a scratch or two in it somewhere - but no dents. What I mean is if the paint is unbroken and adhering completely everywhere with no rust at all. If that is the case all you have to do is sand it with 400p paper and then seal it and you'll be ready to paint. The same thing is true of the side covers. If you wipe all that rattle-can crap off of it with Acetone and find that there is stock paint under it that is in good shape, then just wet sand it with 400p paper and move on. However with the side covers once you get the trash-paint off of it it really doesn't matter what you find below it because you're still going to have to sand them flat and smooth before you seal them.
Back to the tank. If you brought the tank to me I would have it down to bare metal in less than 20 minutes. Put an 80 grit wheel on an angle grinder and you can do it too.
In any event, once you have your old paint off the parts you need to protect them. You do this by sealing them. For a sealer you use a special type of paint that is hard, somewhat flexible, and impervious to other chemicals (except Acetone) and paints but still able to provide an outstanding bond to both the surface under it and the things that will be applied to it. There is only one good product for this - epoxy primer. Go to your local automotive paint supplier and ask for a quart of the PPG Omni line of Epoxy (and catalyst). Depending on what color of it you want (get gray or white) the number will vary but tell them you want MP-170; the gray is designated MP-171 and the white is called MP-172. You can thin this stuff slightly with 10% acetone when you use it for a final sealer coat, but for your first seal to protect the metal just go ahead and shoot it unthinned. After that do your preparation in the normal way. I'll outline it for you:
First you to to a sound base or bare metal.
Next you seal the bare metal to protect it from rust, this is done with epoxy primer.
Next you apply any body filler you might need and sand it to shape.
Next you reseal to protect any metal that was exposed during sanding.
Next you spray multiple coats of high build (2K) primer
Next you block sand the parts to perfection
Next you seal the surfaces prior to painting, here with thined epoxy (10% Acetone) - this primer will NOT be sanded
Next you shoot your base-coat color(s)
Next you shoot your clear coat
Finally you cut and buff the parts
Before you shoot any of the paint products go to the manufacturer's web site and find the Tech Sheets for the product you are using and read it, then follow it. The tech sheet will tell you what you need to know to apply the paint properly. One of the things the sheets will tell you is the size gun cap you need to use. Most of them will give you a wide range but if your gun falls outside of it by a little bit don't worry about it. As an example the Tech Sheet for that MP-170 tells you to use a gun between 1.3mm and 1.6mm, but I've shot it with a touch up gun with a 1.2mm cap for years with no problem.
So by now you're probably thinking one of two things. The first, and most common response, might be "Dear sweet jesus, what have I got myself into here? Maybe I should just pay someone to do this". That might be the smartest thing to do. But then you might have had the other response. That's the one that goes like this, "Hey, I can do this! I've been wanting to try my hand at painting and was just looking for some sort of instructions; this looks easy enough."
You have to make the choice, but let me tell you this to help you along. Anybody can paint. Don't let anyone convince you that its some sort of dark art only learned after years of practice with gawd-awful expensive equipment - it just ain't true. All you have to do is follow the instructions, pay attention to what you're doing, and loosen up - let it flow. Anybody can paint.
