I bought two used steel tanks a few years back, and believe me, they needed a paint job.
First thing I did was figure out how to hang the tank. Obviously I didn't want to hang it by the valve, so I removed that. Keep in mind, once you take the valve off, make sure you tape over the tank opening to keep anything from getting inside of it (yet another great use for duct tape!).
Now, you need to rig something to thread into the tank to hang it by. Most tanks are threaded to at 3/4-14 NPST thread. This is a standard pipe thread, so that lead me to the plumbing section of the local hardware store. There I found a 3/4" reducing nipple and a few other fittings, washers and an eye bolt and nut that fit together and sealed rather well. Once this was assembled, I threaded it into the tank. Here's the part where you have to be careful. Hand tighten the hangar, don't grab the big 2 1/2" wrench and start cranking with that. If you cross-thread the neck, you're done. Hand tighten only here. Finish this off with some careful duct taping or, if you're careful, you can use some silicone RTV gasket maker to seal up the threads and openings on the hangar.
Next step is removing the old paint from the tank. Sanding and scraping can be tedious and labor-intensive, and chemical paint removers like Zip-Strip can be harmful to the person using it, so care must be taken. Luckily, my father had some environmentally-friendly, non-toxic paint remover that he had ordered over the internet. I will get a link from him and post it on here as soon as I can, it's good stuff! So, if you're using that or Zip-Strip, hang the tank from someplace bombproof. I hung mine from a ceiling joist in the garage with a piece of 1/4" polypropolene rope. Wherever you hang it, make sure it's a fairly dust-free environment. I stongly recommend using snap bolt or swivel clip to attach the tank to the rope, or you're going to have to untie and re-tie a lot. Now that you have it in place, apply the paint stripper for the recommended amount of time. Remember to follow the proper instructions for safety equipment and ventilation! Take the tank down and scrape the loose paint off. Repeat as necessary until you've reached bare metal, being careful not to gouge the tank.
To continue preparation, it's time to sand the tank. Even if it's completely bare, a good round of sanding helps prepare the metal to accept paint. I used a pneumatic sander for this, but if you have to hand sand the tank, use some medium-grit emory cloth and sand the entire tank. make sure you get everywhere around the neck, as this is one of the hardest areas to get the paint to bond. Once your tank is sanded down and has a dull luster to it, hang it up and wipe it down thoroughly with denatured alcohol. This will remove any dust on the tank and not leave any residue.
I'd like to take a minute to talk about paint here first. Obviously, you want to use a good quality enamel paint. I prefer Rustoleum, but some people like the Krylon brand. Whichever one you choose, try to stay with the same brand for the primer, paint and clearcoat. For each tank, plan on using one can of primer, two cans of paint and one can of clear coat. There's a lot of selections out there as far as colors and textures and finishes, so get what you like!
Start off with primer, naturally. Give the can a good two minutes of shaking prior to use. Spray the tank completely with primer, turning slowly and covering the metal completely. The bottom of the tank is a little tough to get to so pay special attention to that. Also, watch that you don't get too thick of a coat around the neck. Some people perfer to let the primer dry completely and then wet sand it lightly, but I found that I get good adhesion by not allowing the primer to dry completely before applying a second coat. Put on two light coats of primer to make sure you have complete coverage and keep shaking the can as you go.
Next comes the paint. I waited about ten minutes after the primer coat to start applying color. That was just enough time to let the primer get tacky but not dry. Again, a good two minutes of shaking and use light coats. use overlapping passes and the same precautions as with the primer. Again, I let the paint only get tacky between coats, or you can wet sand again. Two or three light coats should give you some nice, even color.
Finish off with clear coat. I don't wet sand here either, but some people do. Again, same rules and precautions. If you want a deeper-looking finish, only wait a couple of minutes between coats of clearcoat, applying it while it's still wet. This gives a nice, rich, deep finish. Make sure you cover the bottom well and don't get too much buildup around the neck. Allow the final clearcoat to dry at least twenty-four hours before handling the tank.
Now that your paint is dry, take the tank down and remove the hangar from the neck. Since you have the valve off, you can put a new o-ring on it now, or have the dive shop do it when they inspect it. Any time the valve is removed, get your tank visually inspected. Since you have to do this every year anyway, when it's due is a good time to paint. Put on a new boot and you're good to go. Keep in mind that steel tanks need a special boot that has radiused gussests inside to match the bottom of the tank. Send the tank out for an annual inspection and you're good to go.
Remember, tanks take a beating. They get bumped off of things, laid down on the rocks, rolled around in truck beds, and anything else you can imagine. So don't be surprised if you end up doing this every few years if you like having a fresh-looking tank. I personally found that I've ended up doing this every year, not only for the look, but for the ability to change the tank color.
I know there's been a lot of talk about the tank being a pressure vessel and should be handled and treated only in certian ways. Therefore, I've run this method by several coworkers of mine; three mechanical engineers, a structural engineer, two metallurgists and a master machinist. They've all agreed that there is nothing in this procedure that, if done correctly, will harm to or compromise the integrity of the steel tank.
When I do this again this spring, I'll be sure to document it with some pictures. I'll follow up this post with a link to the place that the environmentally safe paint stripper came from. I hope this information is helpful and that it illustrates how easy it really is to repaint a steel tank. Incidentally, I've never had a problem getting a repainted tank filled, inspected or hydrotested.
Safe diving!
Joe
