Guess I would start with what is on the surface right now. Undercoating, paint, grime ....?
Start with some weak simple green type cleaner and work from there. Recently cleaned some - but just a very little grime build up after all these years plus a little overspray from a repaint so had to progress to some enamel reducer to take it off. Finished off with some protective oil and hopefully its protected for a few more years
Just what worked for me