It's a common problem. The usual thought is that the fuel softens the paint and when you fill the pump nozzle chips it and knocks it about. Repainting (as you've guessed) will last about as long as the original paintwork.
The only half decent solution I've found to it is one of
these flush mount gas caps by Kurakyn. They include a chrome sleeve that screws into the tank filler (protecting the paint) which the actual cap screws into. There's been some problems with the newer style tanks but as far as I know the older ones (with the knee indent) should be ok. You need the vented version with the right hand thread.
