I built a concrete benchtop, and sealed it using the “Penetrating Sealer Deluxe” for benchtops. However, I'd like to get a better stain protection, so is it possible to apply a urethane sealer on top of it? If so, is there any special surface preparation that is required?
A film forming coating like the Urethane can be applied over a penetrating sealer providing penetrating sealer has not been highly built. In addition, if a solvent based film forming coating is applied over a water based penetrating sealer, the solvent is best applied by “floating on” or lightly applied not to agitate existing water based sealer. This of course will also depend on how much of existing sealer is present. If concerned existing sealer is excessive you could do a light solvent bath with xylene to remove as much of existing sealer residue as possible.