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So we are refinishing an old oak hardwood floor, we have sanded it and are cleaning it currently before we apply a satin water-based polyurethane (with no stain). Probably from varathane or minwax.

I was watching a couple of videos, and some people use some sort of base coat (I believe they are called sanding sealers). Some say its for the purpose of leveling the floors and making the poly go on evenly, some say its to protect the floor from having the poly discolour it. I'm not 100% sure what this accomplishes or whether or not it's necessary. Information seems to be very mixed.

What does a sanding sealer do? Should I put one on an oak floor for optimal results? I'm not overly concerned about the extra effort.

My current plan is to do a test with and without in a closet, but I think having some more information from someone with experience in this area would help me make a more informed decision.

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I would always suggest you avoid sanding sealers unless you have a specific reason to use one. Sanding sealers place a clear base coat between the wood and finish. But because the sealer is formulated to build fast and be easy to sand it can often be softer than the top coat which is problematic. The only advantage it would offer in your specific case is faster drying which might let you seal, sand & poly all in one day. Using a product you're unfamiliar with that may cause the finish to fail in order to save a day on a project is a terrible idea.

To give you a better idea what sealers (sander or otherwise are for) here's a non-comprehensive list of uses:

  • Lacquer finishes are sprayed on in a lot of thin coats. For woods with large pores that absorb a lot of lacquer sealing them can save time because it's faster to seal and sand than it is to shoot three extra coats before the finish begins to build.
  • Pine knots are infamous for leaking resins that cause trouble with finishes. The Knotty Pine movement is probably far enough in the past that it's not a big issue today, but it may come back. Teak and other oily exotics are in a similar category.
  • You can dye wood if you want a funky color and the grain showing. I bet you can guess what happens if you apply a water base topcoat over a water soluble dye. The problem is best avoided by not using a water soluble dye in the first place, but sealing is an option.
  • Tobacco smoke. In contrast to most finishes, sealers block odors. Animals and exotic cuisine odors can also fit in this category.
  • Nothing ruins furniture quite like silicone furniture polishes. Once absorbed by the wood the silicone ruins refinishing efforts. Rather than risk fish-eye most refinishers will seal everything after stripping.

In general you can do everything a sanding sealer can do with a clear topcoat. You can raise the grain with the sealer and sand it off, or you can sand the base coat. Sanding sealers also get suggested under pore fillers, but you can almost always forgo filling the pores in favor of letting the clear topcoat do it.

The one exception that I can think of offhand are faux finishes, where you want to pass one wood off as another.

  • Awesome, thanks for the information. I think we're going to forego a sanding sealer and go straight with the poly. – Iron_Teapot Jul 28 '17 at 20:54

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