400 Grit is awfully coarse. Lacquer thinner will remove most spray paints and shouldn't hurt the base paint if it's enamel. It will still be tedious work though, and messy. It will need buffed after that.
You can also wet sand with 1500 Grit and follow with a buff.
Agree with ^.
Try lacquer thinner or acetone first.
Even though you didn’t say, it is inferred that you’re not prepping for new paint but rather trying to just get back to what may or may not be good factory looking paint underneath (so you dont have to wear a bag over your head taking that abomination of a camo job out in public!).
The warning.
1. You may not like what you see once you remove the meth spray bomb camo.
2. The same solvents mentioned will also not be kind to old OE single stage 80s paint. (Test this and see how much original paint comes off somewhere with the solvent).
3. There likely isn’t enough mil thickness of original paint to do much more than a quick cut and buff after you get the special paint job removed. And even then, the sunny sides of the truck have a good propensity for burning through.
4. 400 is WAY too rough
No good results guaranteed but I think I would tackle it as follows.
As carefully as your patience allows, try to remove the spray paint like
@bucket said, with solvent. And don’t get ocd about every little bit of it. You’re trying to get most of it or leave what’s left very thin, without softening and eating up the original paint, as much as possible.
Then, rotary polisher, wool pad, med/heavy cut compound, work primarily the areas where some spray paint is leftover.
If you can get all the spray paint off with minimal damage and a half assed shine on the real paint, I’d stop here and consider it a job well done and a success.
Then, if it’s not shiny enough and you think there’s enough paint left to polish. Then do the same with polishing compound.
You may find some areas where a little wet sanding is preferable to get the spray paint off, but I’d be in the 1000 grit range not 400. And know that any high spot imperfections may/will burn through if you’re scrubbing it with paper, before you even notice.
I would test a method on one area.
Preferably below the trim line. This is where the OE paint will still be thickest and in best condition and also least noticeable if you have to abort the mission or re-paint. If it works there, work your way up. Bottom will be hardest to screw up. Sides will be more likely. Flat surfaces will be the most delicate/thinnest.
And if it appears it may work once you’re above the chrome trim line, get that nasty yellow trim off!
Oh and the upper pinstripes will not likely survive this procedure. Hope that you can carefully polish the spray paint off of them. If you are trying to save them. Otherwise don’t try to scrape them off. Only way they’re coming off and not ruining the paint underneath is some scraping and an eraser wheel. Those crusty old pinstripes were stuck better than anything else on my truck. And you can bank on yours being the same.
All said and done, with a lot of care, I give this a 50/50 chance of success unless the spray paint was so graciously applied over very good paint (and it wasn’t judging by the condition of the truck).
Another idea for stubborn areas as I think it would be far too tedious as a primary method is an eraser wheel. It may take that spray pAint off pretty well although I’ve only used them on decals and trim tape/glue/residue.
