I tried to use compressed air to dry auto parts, but it seemed to blow out moist, rusty air! Is this because I live in the humid midwest? I have also stored air compressed in an air cylinder, could this cause rusted, moist air to ruin my otherwise thorough dry job?
Short version is yes. The larger your tank is the less of a problem it is going to be so long as you do a couple of things. First, get a moisture collection device of some type and keep it drained and be religious about checking and draining it.
On my setup I have a 60 gallon tank, then a moisture separator and desiccant collector. After that I feed that output to a second 60 gallon tank that I feed to my air manifold. Usually by the time I check for moisture in the second tank there is none, because the first tank and the moisture separator take care of of it. I'm a little paranoid about moisture at my blast cabinet (since I do powder coating) so I have a third tank setup that I use as a moisture collection tank. It's a dual tank setup that started life as a jobsite compressor that a buddy gave me after the compressor crapped out. Ironically he sand blasted it years ago, never painted or powder coated it, and it's just now starting to get a little surface rust. I probably should take it off, blast it, and powder coat it to make it look all pretty. Needless to say it never has any moisture in it when I check so I figure I am doing a good job of providing dump locations. Usually when I am doing powder or Cerakote I will hook off that tank as well in order to make sure I have CDA.
So barring you do not want to go to those extremes, if you have like maybe a 30 to 60 gallon tank you can fill it and then turn off the power and let it sit for a while to get the temp inside the tank close to ambient air temps. Might take a couple hours but it's one of those things you could do at night before going to bed and then let it sit all night. Once you get up make sure the power is still off and it has not leaked down. Probably a good time to fix your leaks if you have them otherwise its going to be an uphill battle. If it is still pressurized do a quick dump on the moisture drain to get any moisture out, and then once you have done that leave it off and use the air to blow off surfaces sparingly, as much as you can without actually running the pump. Basically do all your work and then blow it off last, and then on top of that get a couple gallons of acetone and some microfiber towels and wipe your work area down with acetone as you are finishing blowing it off. The acetone will help mix with any trace moisture and cause it to evaporate quickly so as to not leave any moisture standing on your bare frame. It also leaves no residue so it's a prep wipe for whatever primer you want to setup for.
If you have a small compressor you are going to keep having the moisture issue but if you can't get a larger compressor then switch to a good etch primer in a spray can if you are down to bare metal, if not down to bare metal then just use a good spray can primer that is high build and made for durable finishes (such as epoxy and what not). It might cost more to go with spray cans but you eliminate a source of moisture and the primer coat is where it's relaly going to help preserve your base metal layer.
