Ask the Staff
Can pure or polymerized tung oil be applied over an existing, fully cured oil finish?
It is a tough question to answer with a broad single statement since their is no "standard" oil finish out there. There are a lot of products labeled as such, but there is no "standard" that manufacturers have to adhere to in calling their products an "oil" finish.
Your polymerized tung oil product over a cured
Watco Danish Oil
, a boiled linseed oil finish or pure tung oil finish would probably be OK. I will assume you plan to wipe on and wipe off your PTO finish and not put on an excessive amount, (thus not expecting it to "flow" or layout" onto the surface in abundance). Wiping on and wiping off, then letting the coat cure would be the best chance of having your application dry OK. You would then need to let that cure fully before applying other coats of your PTO finish.
If the existing "oil" finish is some blend of products and has a significant "build" (maybe the oil finish has a good amount of varnish in it) your product could have adhesion issues. What you want to keep in mind is that "oil" finishes should be based on a solvent of mineral spirits. Subsequent coats do NOT "meld" or soften previous coats for bonding. Oil finishes require a mechanical bond to adhere to one another. This is why you must first sand a varnish (an oil finish) in between coats and create minor crevices for the next coat to better bond to the previous coat. Of course the light sanding removes the slight brushing defects of the previous coat, but the sanding in between coats is critical for the bonding of the next coat.
In oil finishes with very little "solids" (say just linseed oil and no varnish added), a "build up" doesn't really happen and so additional coats could be applied with little to no sanding between coats. So just keep this in mind with your finish going on top of the existing finish.
If you have some inconspicuous place on the piece where you can apply a coat or two of your PTO over the existing finish to see how it behaves and looks, that would be ideal. We are big believers in the mantra "know your methods and know your results BEFORE you start to refinish your mother-in-law's piano bench." This is all the more true when using a finish for the first time and even more important when using something over an existing finish that you do not know precisely what it is.
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