Determine if Coating on Floor is Shellac or Old Polyurethane?

I removed the wall to wall carpet from the 70 year old house I recently bought. The floors in the bedrooms appears to be pine coated with shellac or possibly polyurethane. I don't want to refinish the floors because of the beautiful patina. In only isolated spots and by the door the coating show heavy wear.
How can I find out which coating is on the floors? How can I protect and preserve the floors? What should I use to clean and preserve them?
Thanks,
Mike
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Shellac will completely dissolve in alcohol. I do not know how to identify polyurethane. Don Young

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I'll try some denatured alcohol.
Mike

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You're on the right track with alcohol on a rag in an inconspicuous location. Probably shellac being the age it is. Shellac and lacquer are the only finishes I know of that melt the previous coats making a single coatd of finish on the substrate. Polyurethane, a varnish, doesn't melt previous and requires scuff sanding between coats to get good adhesion.

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90% of the floor looks good and I really don't want to refinish them. If it's poly coated I can sand (which grit sandpaper?) then recoat with poly? Dpes it matter if it's water based poly?
Mike

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How do I know if I need to fresh coat to be oil base dor water based? Does it matter?
Mike
wrote:

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Waterbased can be used as the finish is fully cured. It doesn't smell like oil based does and doesn't have the ambering effect like oil. Scuff sand with 150 grit and wipe up the dust with a rag dampened with water. Don't use paint thinner/mineral spirits around anything that will be topcoated with waterbased finish!

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Do I need to find out if the original poly is water based or oil based before applying a top coat?
Mike

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No, you can apply water over oil. Whether it is oil or water you are covering over you will need to lightly sand. I doubt a 70 yr old floor is poly though. Poly wasn't invented until the mid 50's. What did you find with the alcohol test?
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The house went through a major renovations during the 70s and 80s. It appears the floors have been refinished (power sanded) at once. Haven't tried the alcoholl yet. Going to test this weekend.
Mike
bro says...

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