Q: Do I have to put on a “sealer” after sanding a hardwood floor or is it okay if I just use varnish? And for what reason are people sealing wood floors? Is wood sealer necessary?
A: That is a somewhat complicated answer. It depends what finishes and sealers they are using. If it is a solvent/oil based finish then I would fall back to the older architects recommendation to apply only polyurethane with the first coat cut with mineral spirits by 10% to aid in penetration. Since those days some sealers in this category have been developed which are actually polyurethane but designed as a first coat sealer.
One such product is Poloplaz fast dry sealer. Excellent product. Great penetration and pretty tough in it’s own right. Remember, it is polyurethane. In Toronto where I learned my trade, everyone is using lacquer as a sealer. Really bad idea and will give you a floor finish that will not stand up in the long run. In the water borne category some sealers are designed to help avoid a condition called side bonding and panelization. In this category, a sealer is fine to use. In solvent based, any sealer should be a polyurethane product.
Related Q: After staining Red Oak should you apply a sealer to keep the grain from lifting before applying the urethene?
A: You don’t actually need a sealer when applying stain. However, I do use a polyurethane sealer which penetrates the wood really well from Poloplaz. Before this type of sealer was created architects used to recommend cutting the first coat of polyurethane with 10% mineral spirits.