Oven cleaner, then the "engine degreaser" option at your local coin car wash will do a good job of eliminating what's there. If you want to repaint it use flat, not gloss paint, and spray at about a 45 degree angle so only the edge of the fins gets painted. The fins in the middle need to be left unpainted for the best heat transfer. That's how we did it when I worked part-time at a radiator shop.
Jerry
