refinishing wood floors

OK, I have a house built in the 1930`s with Northern Pine as my bedroom floors. I just finished sanding off all the old finish. My question is do I need to seal the floor before I polyurathane it or is it not necessary to seal before the finish? Also what is a good sealer and polyurathane? Or should a varnish or shellac be used instead of poly? Thanks
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No sealer is needed. If using an oil based poly, thin the first coat with mineral spirits about 20%.
Polyurethane is a varnish. Many floor finishers use the water base as it does not have the odors of the oil based. Water is more clear, oil gives warmer color.
I like shellac, but it is rarely used on floors these days. Poly is usually easier to care for and is very durable.
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Polyurethane can be both your sealer and your final finish. If you're staining your floors first, most stains have a sealer in them. Do not mix poly and shellac or varnish finishes. Poly will be one of the the toughest finishes you can apply if done right (it's what they use in roller rinks - imagine the abuse there!) If you want a harder poly finish consider catalyzed polyurethane (also known as a swedish finish) - however beware that this kind of finish can be difficult to work with. First the fumes can all but kill you, the working time is relatively short and whatever it gets on, it stays on (like tools.) For most folks, catalyzed poly is best left to professionals, but I was able to apply it myself (by carefully following instructions) with great results.
Jeff
mo wrote:

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Since it is Northern Pine and not a hardwood you need a polyurethane that flexes. I once bought Sherwin Williams poly and put it on pine and it was a disaster. You better ask someone experience with pine floors.

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