Paint stripper will work too. Auto parts here sell "Aircraft" paint stripper in quarts, maybe gallons & aerosol I think. About $7 a quart and I've used that to loosen it up. It's been a while but I seem to recall being disappointed with the aerosols.
Are the fenders off the truck? I stripped one of my rear fenders and had one media blasted. The later is allot easier, just find a place that won't use sand or won't warp it.
