What test confirms surface cleanliness and absence of oils before priming?

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Before priming, you must confirm the surface is truly oil-free because any oil or wax will stop the primer from bonding. The best check is to wipe the surface with a tack cloth or use a cleaner/degreaser and then look for any residue. If the wipe leaves a film or the surface still feels tacky, clean again and recheck until no residue remains. Visual inspection alone can miss hidden oils, and wiping with water can leave mineral or soap films. Rubbing with sandpaper isn’t a test for cleanliness and can introduce its own contaminants or alter the surface texture. Using a tack cloth or cleaner/degreaser and ensuring no residue remains directly verifies that the surface is ready for primer adhesion.