Can I use polyurethane over wood that was painted with exterior flat latex paint?

Question

Will it work to protect, or will it not adhere to the painted surface?

Answer 1

Yes, It will give you added protection, to make sure that is adhears to the painted surface, (even if it is flat paint and there is no sheen to it at all, there are still additives that are in the paint that seal the surface) you will need to take a peice of fine grit sand paper, (120 grit or higher) or use a palm sander if you are doing large area, and just "rough up" the surface that you are applying the polyurethane to. (Basicly what you are doing is "opening" up the pores/surface of the painted surface so that the polyurethane adhears into the painted surface and not ON the surface) Also make sure to wipe ALL of the dust and dirt off of the surface too, or your polyurethane will just stick to the dust and dirt and peel off.

Answer 2

You can use polyurethane over almost anything - it will just give an almost clear coat... but if you're going over latex, you need to get a water based polyurethane.