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| polyurethane problem http://www.luthiersforum.com/forum/viewtopic.php?f=10101&t=44596 |
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| Author: | wbergman [ Mon Nov 17, 2014 1:55 pm ] |
| Post subject: | polyurethane problem |
I don't remember this happening before. I made a little box of mahogany, ebony, and IRW. I varnished with polyurethane. The mahogany dried fine, but the ebony and IRW remain tacky after five hours. It's a new can of polyurethane that I also used on an aspen shelf, and it dried fine. Any advice? |
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| Author: | meddlingfool [ Mon Nov 17, 2014 2:01 pm ] |
| Post subject: | Re: polyurethane problem |
The oils in the ebony and rosewood can prevent the poly from drying as fast as the other woods. Give it a few days it'll probably be fine. In future, seal with shop made dewaxed shellac... |
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| Author: | dzsmith [ Mon Nov 17, 2014 7:46 pm ] |
| Post subject: | Re: polyurethane problem |
Yep, I had the same experience with Coco. I ended up removing the finish and just burnished the bare wood with a cloth. Coco does not glue worth a darn either. |
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| Author: | Stuart Gort [ Tue Nov 18, 2014 11:29 am ] |
| Post subject: | Re: polyurethane problem |
I may be the oils in the wood. It may also be exposure to the atmosphere. Prior to being catalyzed, polyurethane is quite hydroscopic and is very reactive, hence susceptible to atomospheric moisture. In production situations one must go to great lengths to store urethane products properly. Could it be that the material was old and stored improperly? |
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| Author: | wbergman [ Tue Nov 18, 2014 3:12 pm ] |
| Post subject: | Re: polyurethane problem |
It was a new can that I had opened a few times within the last two weeks for another project. It dried fine on the lighter colored woods. After another day, it is getting a little better. So, instead of two coats per day, I guess maybe it will be one coat every third day. |
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| Author: | StevenWheeler [ Tue Nov 18, 2014 8:00 pm ] |
| Post subject: | Re: polyurethane problem |
The curing of a polyurethane finish occurs by oxidation. Rosewoods and some other oily woods contain anti-oxidants, and therein lies the problem. As Ed stated, dewaxed shellac is your friend in this situation. I use two coats of Zinnser Seal Coat before applying polyurethane to known culprits. |
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