Several years ago a friend of mine built a UDS from a 55 gallon drum that was painted on the inside. He made all of his cuts and installations, then built a roaring fire in it. Next day, he was able to scrape out the inside (messy job) and get almost all the paint as ash and peelings. He then repeated the roaring fire two more times then gave it a final scrape down, using a wire brush to get into the corners. He's been cooking on it for years now and turning out great food. I think the type of paint (in this case not high temp) will have a lot to do with it if you want to go this route.
Hub