Shellac used to be the traditional floor finish. That was before the advancement in research and technology that improved finishes for floors under the urethane family.
Shellac is a beauty for wooden floors but it’s not as durable as poly. If you have a shellacked floor you may wonder if water-based poly can go over your shellac.
Related Article: Shellac vs Polyurethane on Floors
Water-based polyurethane over shellac coat
Water-based poly can be used on top of shellac provided the shellac is dry and clean. A slight sanding may be necessary for mechanical adhesion between the two coats.
Water-based polyurethane will be a good choice for lighter woods or in instances where you need not alter the appearance that the shellac is giving your floorboards. Oil-based polyurethane has a tendency to yellow over time something that you will not experience with water-based polyurethane. Here are more details on applying poly over shellac.
Dealing with blemishes before applying poly over shellac
Water-based poly cures to a clear coat. This clear coat can easily telegraph imperfections of the previous layer and make them conspicuous.
Therefore any blemishes should be dealt with before applying polyurethane. Sanding with 300+ grit sandpaper will take care of bumps and any stains on the floorboards.
After sanding it is ideal to clean and tack your floor to get read of dust and any other suspended particles that may interfere with the polyurethane coat that should be applied next.
If you are unsure how to apply water-based poly follow the link to see how to do it the right way with the right tools.
Shellac and polyurethane adhesion
Waxed shellac is said to have adhesion problems with poly. But are these claims legit? Dewaxed shellac is usually labeled as ‘compatible with all finishes’.
Below is a video that checks these claims through an experiment. In the investigation, Marc Spagnuolo uses three boards coated in poly, dewaxed shellac, and waxed shellac. All boards are decorated with 2 coats of poly and after it cures for about a week tests are run to see if there are any adhesion problems.
