Removing a single coat of mystery finish?

[Justin Stephen]

So I have a segmented bowl that I put one coat of finish on some time ago but never finished up. I *think* the finish was WOP, but I am not 100% certain. I now have a reason to finish it but was going to use an oil finish (probably AO or DO). I've never had this problem before and wanted to make sure I do this right. As I recall, putting oil over WOP doesn't work well but I could be wrong about that.

Will a rigorous sanding with 320 remove the WOP (or whatever it is) well enough to safely put an oil finish on afterwards?

Thanks in advance for any advice.

[John Keeton]

You will clog a bunch of sandpaper trying to sand off the finish. Were it me, I would use a citrus stripper. All that said, the AO and DO are both blends of oil and urethane resins, as I recall, and that is pretty much what WOP is - perhaps with a different ratio. I am not sure what you will gain, but apparently you have a plan.

[Reed Gray]

I am fairly sure that most of the penetrating, solvent based finishes are similar enough that you can put one over another. The biggest difference is how much resin is in each. The Deft oil has urethane resins in it, and used to be my staple for flat work. The Watco didn't have urethane. I would suggest you apply a spot and see what it does. You may want to rough up the surface with some fine grit abrasive first. If you were going to try to put it on over a walnut oil and wax finish, the solvents would remove the wax, and the oil would penetrate as well.

robo hippy

[robert baccus]

urethanes are a bear to repair or recoat or refinish.

[ALAN HOLLAR]

WOP is just thinned polyurethane varnish, with no penetrating component. It will act as a sealer, preventing oils from penetrating into the wood. Citrus or methylene chloride strippers should take a thin coat off easily if messily. Sanding a thin coat off is not too hard but 80 or 120 grit would be a better place to start. 320 grit will load almost immediately, and then just generate heat and frustration. I spent a lot of years refinishing almost everything under the sun, and have always found a chemical stripping to be easier on the wood surface, and less likely to create a situation regarding heavy sanding which can blur or remove fine details.

[Justin Stephen]

Thanks all!

[Justin Stephen]

WOP is just thinned polyurethane varnish, with no penetrating component. It will act as a sealer, preventing oils from penetrating into the wood.

That's what I had thought I had read here before, that you could basically put WOP over an oil or oil/varnish finish but not the other way around. Honestly, I've never done a finish before where I have mixed the two.

[Reed Gray]

Alan,
A question or two... I always have thought that any varnish and solvent blend was a 'penetrating' oil, so, it would soak in and then cure. If you wanted a more shiny finish, you have to apply multiple coats, and eventually, after what has penetrated the surface has cured, then it starts to build up on the surface. The other method was to use a sanding sealer, then every coat built up on the surface. The only ones I know that didn't penetrate the surface are the water based finishes. Am I even close? It has been a while since I read Bob Flexner's book...

robo hippy

[Rob Price]

Polyurethane is a plastic polymer. The oils and solvents in it do penetrate some but not the polymer. once the finish cures you have a plastic film on top of the wood. Various commercial finishes will mix in other oils and decrease the solids content of the polyurethane and create less of a film. Most WOP are just thinned polyurethane. But any finish on top of that plastic film will not reach the wood. Whereas a pure oil finish will soak into the wood, and future coats can soak in as well. Eventually you will get some build but it takes much longer.

I've mixed BLO and poly to create my own 'penetrating oil' but it still builds. Just slowly.