Re varnishing oak. Oil based poly over ?

I need to apply some new finish over a light oak finish on an old desk. My favorite choice is some old oil based polyurethane (varathane) I already have. Unfortunately, I don't know what the origional finish was. I sanded the old finish to remove the problem spots, and would like to just coat it with my poly. Is it likely that the origional finish will cause problems with the oil based urethane?
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It is not always safe to put poly over lacquer. So if the current finish is lacquer then you may have a problem.The problem is that lacquer can dissolve from the solvents in the poly and then the lacquer dries faster than the poly and it makes wrinkles. This is the same principle they use with crackle paint. However, sometimes the lacquer won't dissolve depending on the solvents in the poly. You really never know in my experience.
1. You can try a small test area. Best if you try it on an area where the original finish is pretty thick still so the problem will show up for sure. What you will see if there is a problem is the poly will start to wrinkle up after just a few minutes if it is lacquer underneath. This doesn't always happen but it is a real possibility. or 2. You can apply a coat of dewaxed shellac as a barrier layer, then do the poly. or 3. Make sure you have sanded it pretty well to minimize the amount of lacquer left.
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No, no problems are likely. If you've sanded, there may be bald patches, though, so re-sealing (with shellac) would be an appropriate first step. It all has to be CLEAN first, of course. And, the writing surface should be filled if the oak is open-pored.
It's midwinter here; are you going to be applying an oil-based finish indoors?
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Bob F wrote:

No.
1. Your desk - unless you finished it - is most likely lacquer. The solvent in oil poly is mineral spirits. That won't affect lacquer. Going the other way - lacquer *over* poly - may well mess up the poly.
2. Shellac is another (not likely) existing finish. Shellac will go over anything and can be topped with anything.
3. A third possibility is alkyd/phenolic varnish. Same deal as #1.
--

dadiOH
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[...snip...]

Yes, but ... if the original shellac contained wax, the poly may not adhere well. So a new washcoat of dewaxed shellac is still in order.
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How old? If it's over 2 years old or if it looks "seedy," toss it and use fresh. Your work is worth much more than a $10 can of finish.

Try a coat. If it looks spotty, sand it back to bare wood and start anew.
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Good move. Should work out fine.
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