When should you apply top coat?

You should apply top coat after primer, undercoat, or base coat has dried properly and surface has been prepped. For interior and exterior house painting, apply top coat after primer or undercoat has dried according to label, typically 1 to 2 hours for water based primer, 24 hours for oil based primer, then lightly sand to smooth imperfections, wipe dust, and apply first top coat when temperature is 50 to 85 degrees Fahrenheit and humidity below 70 percent, then let dry 2 to 4 hours for latex and apply second top coat. For woodworking, apply top coat like polyurethane after stain has dried fully, usually 24 hours, and after sanding smooth. For automotive painting, apply clear top coat after base color coat has flashed, usually 15 to 30 minutes after base, while base is still slightly tacky for chemical bonding, not after fully cured. For nails, apply top coat after color polish has dried slightly, about 2 minutes, so color is set but not fully hard, to seal and add shine. For concrete sealer, apply top coat after concrete cured 28 days. So apply top coat when previous coat is dry to touch but within recoat window, surface is clean, dry, smooth, and conditions are moderate, not in extreme cold, heat, rain, or high humidity, and when you want final protection and finish.