I understand the possible problem painting at a public airport but what about renting a hangar at a private airstrip (like mine)?
Many (all?) states have rules on painting regardless of where you do it. For example, the state of Iowa will allow you to do some painting for personal use with no oversight. When you exceed a set amount of paint being applied within a year the department of natural resources gets involved even if you aren’t doing it for a living. They’re immediately involved with things when it is a business.
A paint shop or mechanic that is painting could potentially fly under the radar for a while at a private airport but I suspect that any legitimate business will want to do things the right way to avoid the potential for fines.