1. All air compressors should have at least a water trap combined with a regulator
. It a safty issue as you can overload you air tools the cheap
ones can explode or at least fail. So don't use an air compressor without one. Like the other post side drain the air tank I got 11 bucket from one compressor and I wasn't ever at the place for that job. The bloke bought a air receiver dryer to use with his sensitive equipment
and didn't think he needed to drain the air tank. Self main millionaire by the way.
2. We ever I paint an engine I have used high temp paint in a airesole can. Just before I have gone over the block (that is clean) with a propane
torch, as you move across the cact iron you can see moisture drying off from the surface. And then while it as still a bit warm I apply the paint, so its warm but not hot enough to burn you or the paint. I have never had a problem. The cast is porous on the outside. Be careful don,t set fire to anything.
3. I would look for a leak or drip maybe. It it generally surface rust all over or just in one area.