Polyurethane over mineral oil? - Woodworking Talk - Woodworkers Forum

 
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post #1 of 7 Old 02-25-2012, 01:28 PM Thread Starter
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Polyurethane over mineral oil?

I have a butcher block desk top that has so far only been finished with mineral oil. Unfortunately we haven't applied the oil often enough and the desk has some very slight cracks between a few of the boards. I want to fill the cracks, sand away the roughness and finish with a longer lasting sealant (polyurethane). I noticed the underside of the desk top, which has had no wear and tear looks much richer and the grain is darker and nicer looking; the top however looks worn and lighter (I assume the oil has been worn away). I'd like to re-apply some mineral oil to bring out the natural grain and improve the look of the wood before I apply the polyurethane.

Is it ok to put mineral oil underneath a water-based polyurethane? Is there a better way to enhance the natural grain?
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post #2 of 7 Old 02-25-2012, 10:01 PM
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Originally Posted by ekallevig View Post
I have a butcher block desk top that has so far only been finished with mineral oil. Unfortunately we haven't applied the oil often enough and the desk has some very slight cracks between a few of the boards. I want to fill the cracks, sand away the roughness and finish with a longer lasting sealant (polyurethane). I noticed the underside of the desk top, which has had no wear and tear looks much richer and the grain is darker and nicer looking; the top however looks worn and lighter (I assume the oil has been worn away). I'd like to re-apply some mineral oil to bring out the natural grain and improve the look of the wood before I apply the polyurethane.

Is it ok to put mineral oil underneath a water-based polyurethane? Is there a better way to enhance the natural grain?
Mineral oil is not a drying oil, fine for butcher blocks that food is being prepared on, not good for a home desk work surface and not necessary. You have a few choices as to "drying" oils to take its place, tung, linseed oil [boiled] are the main two used though BLO darkens with age. Tung darken when applied but once it darkens it does not continue to darken. The other would be dehydrated castor oil which is not generally available to the home owner. there are a few more but with the same unavailability as to ease of local purchase.

you should not apply poly over a non drying oil. my advice would be to srub the surface with alcohol and then lacquer thinner a few times and then sand smooth and apply one of the two readily available oil, let dry [ a week or more for tung], and then apply your poly.

If for any reason your paper clogs up when sanding, that would be an indication that oil is still present in the wood.
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post #3 of 7 Old 02-25-2012, 10:14 PM
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use an oilbased polyurethane . it will enhance the color much better than water based. If the surface is not oily still with the mineral oil there will not be a problem.
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post #4 of 7 Old 02-25-2012, 10:48 PM
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Mineral Oil is sometimes called parrifin oil so I don't believe any chemical coating can be put over it. You will need to strip it with lacquer thinner or a wax and grease remover, perhaps a paint and varnish remover. I would try to get the top where it doesn't feel like it has wax on it anymore. Since it has soaked into the wood it may take many treatments before it could be finished. When you get it ready I would suggest sealing it with a dewaxed shellac before using the poly. Shellac is often used to seal pine knots for finishing.

Last edited by Steve Neul; 02-25-2012 at 11:23 PM.
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post #5 of 7 Old 02-26-2012, 01:10 AM
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As all have said, Lacquer Thinner and a lot of it. You will need to "float" any remaining mineral oil out of the surface.

BTW - If you are not in California, Minwax Antique Oil Finish will darken and seal the surface nicely. (It dries a lot quicker than BLO.)

Use the right tool for the job.

Rich (Tilting right)
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post #6 of 7 Old 02-26-2012, 05:25 AM
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I agree with the suggestions of removing any oil with lacquer thinner first. If your intent is to try to enhance the grain before topcoating, I would use 25% BLO in VM&P Naptha. Once dry, it can be topcoated with most any film finish.







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post #7 of 7 Old 03-01-2012, 12:22 PM Thread Starter
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Thank you all for these tips -- good to know that I'll need to get rid of the mineral oil before sealing with poly. I appreciate the help!
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