# Polyurethane over mineral oil?



## dougan (Dec 19, 2013)

I am making a cutting board and coaster set out of maple and walnut. I have finished the cutting board with mineral oil which has given me a food-safe, finish that has given a great contrast between the maple and walnut. 

For the coasters, I want something a little more waterproof, and ideally I'd put polyurethane on it since the coasters don't need to be food-safe like the cutting board. However, I want the colors to match perfectly, and to do that I would like to also treat the coasters with mineral oil just to match the coloration of the cutting board.

Would it be OK to put polyurethane over mineral-oil-treated wood? Luckily, if nobody knows, I'm just using scrap wood for this so it give me an opportunity for an experiment. But if someone knows, it'd save me the trouble.

Thanks!


----------



## Fred Hargis (Apr 28, 2012)

I'd be interested in what your trial proves out. I'm betting it won't work, mineral oil doesn't cure like the drying oils. I have to think the varnish isn't going to stick to it.


----------



## adot45 (Jul 8, 2013)

Here is an older thread on that very topic. http://www.woodworkingtalk.com/f8/polyurethane-over-mineral-oil-36195/


----------



## Rick Mosher (Feb 26, 2009)

Just do some samples on scrap. I would be surprised if the polyurethane only looks any different than the mineral oil. The only way it would be a different color would be if you used an amber varnish. There will be a sheen difference though unless you use a Flat sheen urethane.


----------



## HowardAcheson (Nov 25, 2011)

>>>> ideally I'd put polyurethane on it since the coasters don't need to be food-safe like the cutting board.

Poly and all finishes are required to be non-toxic when cured so you can use any store bought finish you want.

However, poly will not adhere well to mineral oil. I would not use it on top of a surface coated with mineral oil.


----------



## rrbrown (Feb 15, 2009)

I may be wrong but you need to let the oil dry a few days then you can seal it with a thinned cote of shellac. The poly should adhere fine after that. 



This is what was told was fone on a recent project. It was a pil but not sure which one.


----------



## rrbrown (Feb 15, 2009)

Rick Mosher said:


> Just do some samples on scrap. I would be surprised if the polyurethane only looks any different than the mineral oil. The only way it would be a different color would be if you used an amber varnish. There will be a sheen difference though unless you use a Flat sheen urethane.


Oils are used to bring out the grains that a straight finish like poly may not do. The trick is supposed to be hiving it ample dry time then sealing it. I've just learned this stuff but shellac is used to seal before applying many finished. Including water base poly over oil, laquer over oil etc. 

I trust the info I got but hopefully I remembered it correctly. :thumbsup:


----------



## Rick Mosher (Feb 26, 2009)

Maybe I am crazy but as long as it is "oil based" polyurethane you ARE adding oil to the wood. :icon_smile:


----------



## rrbrown (Feb 15, 2009)

Rick Mosher said:


> Maybe I am crazy but as long as it is "oil based" polyurethane you ARE adding oil to the wood. :icon_smile:


You may be right or crazy, I don't know which one or maybe both.:laughing:

Seriously it makes sense but I'm not sure you get the same penetration of oil into the grain as if it was just the oil. It also could be the type of oil. Like I said just learning myself and throwing what I think I know out there to help or learn something new.

I've always hated finishing because there is so many types and ways to get different looks or finishes.


----------



## Steve Neul (Sep 2, 2011)

I'd have to agree with Howard and Rick. Polyurethane inherently has adhesion problems anyway and to try to apply it over mineral oil is asking for the finish to fail. The appearance might be slightly different using a flat oil based polyurethane but it won't be drastically different. I would just use that instead of putting mineral oil on first.


----------



## HowardAcheson (Nov 25, 2011)

>>>> I may be wrong but you need to let the oil dry a few days

That's the problem, mineral oil will never dry. It's not a drying oil. Anything you put on top of it will not adhere well.


----------



## Rick Mosher (Feb 26, 2009)

> mineral oil will never dry. It's not a drying oil. Anything you put on top of it will not adhere well.


Except shellac. Putting mineral oil on the wood is the first step of a french polish finish. The shellac is applied over the wet mineral oil and it all becomes part of the finish.


----------



## cabinetman (Jul 5, 2007)

I've never applied an oil base film finish (or WB poly, or lacquer) to mineral oil treated wood, but I suspect there will be an adhesion problem. Whether mineral oil dries or not, is a matter of conjecture. I have a butcher block counter that has been used for a cutting/preparation board. I oiled it as needed. It got to the point of being dry, and then being re-oiled. It did dry and I can't say where the oil went, but it just went away. Nothing rubs off on a paper towel. It just feels slick like glass. Liquids don't penetrate.

There's a lot of discussion on forums about the barrier properties of shellac, de-waxed to be more specific. While it may work, IMO it's a poor base for a film finish. It's been suggested to use Seal Coat instead of waiting for one finish to cure before adding a topcoat. I just don't agree with putting a finish on an unpredictable surface. I like using like or compatible bases with topcoats.

I think if I wanted to put a film finish over a mineral oil treated wood, I would strip the wood as best as possible. It may take a chemical stripper, or a solvent wash with lacquer thinner repeatedly until its suitable to continue. At that point it may be necessary to use a dye or stain to get the coloring to match other pieces (if that's the case), and the wood will be more prepared to accept a film finish.


















.


----------



## Fred Hargis (Apr 28, 2012)

Rick Mosher said:


> Except shellac. Putting mineral oil on the wood is the first step of a french polish finish. The shellac is applied over the wet mineral oil and it all becomes part of the finish.


I have to admit I use mineral oil in french polishing, but not that way. I use it to lube the pad during the "bodying process". It gets cleaned off as the last step.


----------



## Rick Mosher (Feb 26, 2009)

There are as many ways to do French Polish as there are finishers. Classic Frech Polish is a wax only finish where the wood is burnished and wax is applied. 

One of the classic shellac methods is to apply mineral oil or olive oil to the raw wood, burnish and start applying shellac with a tampon while sprinkling pumice on the wood. The mineral oil becomes part of the finish.

Tage Frid also does what he calls his 4F finish where shellac is applied over wet danish oil or linssed oil and then taken off with steel wool. These aren't techniques I would use but they are valid finishes.

Like everything else you read on the internet don't just blindly believe it, try it yourself and share your results with us. :thumbsup:


----------



## Fred Hargis (Apr 28, 2012)

I agree about the internet misinformation, I also agree about the many ways to do things. But thanks, I have a method that really works well, see below.....sometimes some folks believe hobbyists have no real experience/knowledge so i try to indicate if it's something I've actually done.


----------

