I just had the same problem and this is how I solved the problem
I removed the rust with a wire brush. and sanded it by hand with 400 grit wet sand paper. went to the local auto parts store bought plasti coat auto paint. Not enamal paint (oil base)
This is the paint they use to match the factory paint on the cars.
choose a color that comes as close to the tank color as possible.
I did about 7 coats of primer wet sanding betwwen each coat letting each coat dry completely before the next coat is added. Most IMPORTANT is all the rust must be gone. If not you will have a problem later. After the primer has dried completely I did about 7 coats of the main color also wet sanding between each coat so as to get good adheartion to the previous coat. When your all done painting let it set in a warm place for at least a week to dry completely. After that you can put the boot back on and if you did it right have no more problems. If you get nervous you can have the tanks Hydroed again. Thats what I did. But the bottom of the tanks are the thickest part of the tank. As long as you only wet sand and wire brush the tank and there are no deep pits you should be fine. DO NOT USE ANY POWER TOOLS ON THE TANK. NO GRINDERS OR POWER SANDERS.
Remeber always remove the boot after diving and rinse and dry the tank before storing it. Leave the boot off if possible till the next time you go diving. The rust was caused because the boot was not removed.
I removed the rust with a wire brush. and sanded it by hand with 400 grit wet sand paper. went to the local auto parts store bought plasti coat auto paint. Not enamal paint (oil base)
This is the paint they use to match the factory paint on the cars.
choose a color that comes as close to the tank color as possible.
I did about 7 coats of primer wet sanding betwwen each coat letting each coat dry completely before the next coat is added. Most IMPORTANT is all the rust must be gone. If not you will have a problem later. After the primer has dried completely I did about 7 coats of the main color also wet sanding between each coat so as to get good adheartion to the previous coat. When your all done painting let it set in a warm place for at least a week to dry completely. After that you can put the boot back on and if you did it right have no more problems. If you get nervous you can have the tanks Hydroed again. Thats what I did. But the bottom of the tanks are the thickest part of the tank. As long as you only wet sand and wire brush the tank and there are no deep pits you should be fine. DO NOT USE ANY POWER TOOLS ON THE TANK. NO GRINDERS OR POWER SANDERS.
Remeber always remove the boot after diving and rinse and dry the tank before storing it. Leave the boot off if possible till the next time you go diving. The rust was caused because the boot was not removed.