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Lumber & Composites

Oil Poly over Water Spar

I recently put up pine tongue and groove on the ceiling under my deck. I used a water semi gloss spar as I was worried an oil would have to much amber color. However the wood is just too light. What's the best way to apply an oil color on top? Is it fine to sand it down and reapply an oil to get the water spar layer off? There are two coats but doesn't appear to be very heavy. Any guidance would be much appreciated. 
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Posted 2017-09-10T16:14:14+0000  by ErikOps ErikOps
Hi ErikOps,

You cannot apply an oil based product over a water based product, thus you will have to sand off the original finish and start over. Tints are available for polyurethane finishes, but they are tricky to use to get the exact color you desire, its a trial and error process.  This will take some work, but it can be done. 

Since it's on the ceiling, I would live with the existing color, clear finishes tend to darken over time, give it a few months and then decide on your course of action.

Posted 2017-09-11T19:35:07+0000  by Mike_HD_OC
I would disagree with the statement that "you cannot apply an oil based product over a water based product". I would, however, stay with an oil urethane spar varnish, as spar varnishes, whether oil or water based, are designed to have a more flexible film than conventional urethanes, which are quite brittle. This is important with exterior applications as they are subjected to extreme temperature ranges  and humidity ,which can also be quite rapid.

Having fully cured, a scuff sanding with 220 sandpaper would be advisable for better adhesion of the new spar varnish. before applying the new spar urethane.

Oil based urethanes do have a slight amber tone initially and will probably get more so with time. Also, as Mike indicated in his post, wood will darken and redden with age, even under a coat of varnish. One of the reasons for staining wood, is not just the initial color, but the stabilizing of the color with age. For instance, if you had wanted to keep that wood its light color, a white pickling stain would have prevented it from turning darker and redder with age.
Posted 2017-09-16T01:46:29+0000  by ordjen
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