Can I use shellac as a primer when refinishing?

Q: I’ve never used a shellac primer for wood floors, but in a quick search online, I gather it can be used as a “primer” when refinishing wood floors (to hide any problem areas). I didn’t catch in reading, whether the (oak) wood grain will still show through? Or whether oil polyurethane will adhere to it without problems?

I have 5 rooms to do; two each have one spot of oil/a greasy substance/unidentified. Can I spot prime in this case, or do I need to cover the whole floor with the shellac? And will there be a noticeable difference in the finish from one room to another (being in some I don’t use shellac)?

Perhaps you can offer advice on another possible issue when doing the floors this time of year: I have forced air heating and am concerned that it will blow dust on my wet finish. Can I tape cheesecloth (no laughing, please, if I’m way off here) or some such over the vents to prevent that?

A: I haven’t used shellac either, and am going by comments from some who have used it on a couple of forums I belong to. It would have to be de-waxed shellac. And yes, the polyurethane will adhere to it. It dries very quickly, say half an hour. When it is dry, apply the poly. Don’t buff it. If there are any rough spots or irregularities, wipe with alcohol. Yes, the grain will show through. You should still try to remove this grease or whatever it is and if you are able to accomplish that and the wood is not exposed or without finish, I would then buff and apply a coat of poly. If you are using different finishes from one room to the next, I would expect to see differences in the final product.

If you vacuum out the registers you should be fine as to dust contamination. Shoving something in the vents can help, but it is best not to have the furnace blowing while the finish is setting up anyway.