I recommend only Mr. Muscle brand oven cleaner. The other brands don't contain enough lye. Be very careful with it. It is neutralized with water. If you get it where you don't want it to be, just wash it off. I spray it in a cup and dab it on with a brush or Q-tips. Wait a few minutes and wipe off with water. Apply as many times as necessary. I use a paint scrapper with a plastic blade to help remove the lettering and use Bob Ami cleaner to scrub off the stubborn spots. Then buff it out. Don't spray the entire door and don't leave it on for more than a few minutes at a time. If left on too long it can damage the paint underneath by softening it, leaving a discoloration or removing it completely. It is best to test the background paint somewhere where you will not notice it before attempting to remove the lettering just in case the vehicle's paint job has been poorly done. Sometimes you will have ghosting from the old lettering depending on how long it was on. I always tell the customer what might happen and make sure they understand the risks. In the case of this local fire company, repainting was not in the budget so they took the chance. It turned out great and the ghosting was so faint you didn't notice it with the new lettering. The truck had been repainted several times during it's life.