I resently repainted the tin roof on my Gothic style farm house. (very steep pitch) First, I power-washed the complete roof with a 2500 psi washer. I did scrub some areas with a stiff brush and warm sudsy water, the section under a tree. I purchased the best paint I could find, I didn't want to do this again. Benjamin Moore has a paint formulated for metal. I applied two coats with a roller, using a brush where necessary. Be careful, I customized a wood ladder to have legs to go over the top of the roof and wore a saftey harness.