On cars with paint flaking off I use a single edge razor blade in a scraper. It takes about 20 minutes to do a panel this way (hood. fender, etc.) It doesn't work where bodywork was done. It takes a little practice to get the blade at the right angle and not dig in to the factory paint.

I accidently stripped a quarter panel on a 58 Impala with a pressure washer so that might work on your truck.

Both these methods will only get down to the original paint. To get stripped down to bare metal I use aircraft paint stripper. Always use rubber gloves and eye protection with this stuff and a respirator if you don't have good ventilation.
AL