Do you sand before applying top coat?

Whether you sand before applying top coat depends on surface and previous coat condition, but generally yes, light sanding before topcoat improves adhesion and smoothness. For interior and exterior house painting, if you applied undercoat or primer, you should lightly sand with fine 220 grit sandpaper to smooth any bumps, drips, or rough spots, then wipe dust off with tack cloth before applying topcoat, as this ensures smooth finish. If previous topcoat is still within recoat window, usually 2 to 4 hours for latex, you can apply second topcoat without sanding. If previous coat has dried more than 24 hours, is glossy, or has imperfections, light sanding is recommended to degloss and provide key for new coat to grip. For woodworking, you should always sand lightly with fine grit between topcoats of polyurethane, varnish, or lacquer to smooth and ensure adhesion. For cars, you wet sand base coat lightly before clear topcoat only if needed to remove dust nibs. For previously painted walls in good condition, light sanding to degloss is recommended before topcoat. So yes, you generally sand lightly before applying top coat if surface is rough, glossy, or dried beyond recoat window, but not needed if recoating quickly within recommended time and surface is smooth.