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?

Is it likely that the origional finish will cause problems with the oil based
urethane?

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.

On Jan 25, 10:26*am, "Bob F" wrote:
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?

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?

On Jan 25, 1:26*pm, "Bob F" wrote:
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.

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.

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?

Try a coat. If it looks spotty, sand it back to bare wood and
start anew.

[...snip...]
2. Shellac is another (not likely) existing finish. Shellac will go over
anything and can be topped with anything.

[...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.

Father Haskell wrote:
On Jan 25, 1:26 pm, "Bob F" wrote:
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.

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.

It's probably at least 10 years old. I used it recently on another project, and
it worked fine.


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?

Try a coat. If it looks spotty, sand it back to bare wood and
start anew.

I tried coating a spot on the underside, and it seemed to work fine, so I'm
going to go ahead with the project.

Thanks everyone for the suggestions.

I tried coating a spot on the underside, and it seemed to work fine, so I'm
going to go ahead with the project.

Thanks everyone for the suggestions.

Good move. Should work out fine.