For goodness sakes, we're talking about asbestos, not uranium. The only thing wrong with asbestos shingles on the outside of a house is that they're ugly. They also hold paint like you won't believe. I wouldn't prepare at all. In fact, the only trouble you might have with this stuff is if you sand it or wire brush it. If they've never been painted then you don't have to scrape them. So there you are. Your asbestos shingles pose no danger whatsoever. Just wash and paint.