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Hot take: stop sanding to 600 before you prime
A guy brought his '19 F-150 into my shop in Reno last month after he'd blocked the whole bedside down to 600 and the primer wrinkled up like a raisin under the sealer. He swore the internet said finer grit means a smoother finish, but all that did was polish the panel so the primer had nothing to bite into. Am I the only one who still cuts at 320 and lets the primer do the leveling, or is everyone really finishing before they even start?
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the_rowan8h ago
Going past 320 is a waste on most jobs and you're right that it costs you bite. Where I'd take it further is the grit you stop at depends on what's going over it, not some blanket rule. A sealer like epoxy wants a coarser scratch than a straight primer surfacer does because it needs a mechanical tooth to grab, and polished 600 gives it almost nothing. The guys who blocked to 600 and then got wrinkles did the same thing people do with paint, they treated the last step like the finish when it's really the base. What I tell people is think of the primer as the leveling coat, so leave it a scratch it can dig into, then let the primer and your sanding of the primer get you the smooth you're after. If you finish the metal like it's already painted, you just built a slick surface and then act surprised when nothing sticks.
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