Fully cured latex paint is durable, but when you need extra impact resistance -- for example, on a kitchen cabinets or a floor painted with latex enamel -- one or two coats of polyurethane can provide it. Both oil- and water-based polyurethane products bond well to dried latex paint, and both provide a plastic coating that enhances the sheen as well as providing extra protection.
Let Fresh Paint Cure
It's important to wait for the latex paint to fully cure before you coat it with polyurethane, or the latex undercoat may not harden fully. Although latex paint dries to the touch in a few hours, it can take up to 30 days to cure, depending on the color and thickness of the coating and the ambient temperature and humidity. As a rule of thumb, it's best to wait at least 14 days before coating fresh latex with polyurethane.
Preparing and Finishing
The latex finish you're about to coat with polyurethane must be clean and de-glossed to ensure maximum adhesion of the clear finish.
Things You'll Need
Step 1: Clean the Painted Surface
Wash old latex paint with a strong detergent, which not only removes oils and dirt, but etches the paint. A good all-purpose cleaner consists of 1/2 cup of trisodium phosphate in a gallon of warm water. Scrub lightly with an abrasive sponge.
Step 2: Scuff With Fine Sandpaper
Hand-sand the cured latex finish with 220-grit sandpaper to give the surface "tooth." If you're coating a floor or large vertical surface, make things easier for yourself by attaching the sandpaper to a pole sander.
Step 3: Brush With Long, Even Strokes
Your paintbrush matters: Use a natural-bristle brush for oil-based polyurethane and a synthetic-bristle brush for water-based products. Apply the clear finish in long strokes and finish on the wet edge of the material you've already applied. Look for bubbles after every stroke and brush them out.
Step 4: Scuff Between Coats
Ensure a smoother finish by scuffing the first coat with 220-grit sandpaper after it dries tack-free and before applying any subsequent coats. Do not scuff the final coat.
The durability of polyurethane increases with the sheen. If you want maximum protection, use a high-gloss product.