When to apply topcoat?

When to apply topcoat depends on surface and previous coats, but general timing is after primer and undercoat have dried and been prepped. For interior and exterior house painting, apply topcoat after primer or undercoat has dried according to label, typically 1 to 2 hours for water based primer, 24 hours for oil based, then lightly sand to smooth any imperfections, wipe dust, and apply first topcoat, let dry 2 to 4 hours for latex, then apply second topcoat. Topcoat should be applied when temperature is 50 to 85 degrees Fahrenheit and humidity below 70 percent for best curing. For woodworking, apply topcoat like polyurethane after stain has dried fully, usually 24 hours, then apply thin coats with drying time between. For automotive, apply clear topcoat after base color coat has flashed, usually 15 to 30 minutes after base, before base fully cures, for chemical bonding. For nails, apply topcoat after color polish has dried slightly, about 2 minutes, to seal color. For concrete sealer topcoat, apply after concrete cured 28 days. So apply topcoat when previous coat is dry to touch but within recoat window, surface is clean, smooth, and conditions are dry and moderate temperature, not in extreme cold, heat, or high humidity.