I used aircraft stripper and a pressure washer. Lay it on heavy, let it work and blast it off. This done before exterior paint work... not recommended if the exterior is already painted as any stripper that "splashes" on good paint may damage it. A couple rounds got most of the paint off. Final touch was a stainless scrub pad + stripper to get the last of the factory primer off. Follow with lacquer thinner, sand and prime.

If the exterior is already painted, I'd probably use the cup wire wheel + sandpaper then prime and paint.

It will be time somewhat consuming and dirty either way you do it.