I'm working on some Black Walnut and have been running into a little problem.
I sanded up through the grits to 220, usually pretty decent for this hard of a wood. I then applied a natural tone stain (nothing fancy-Minwax-Natural). After that dried I put on a layer of semi-gloss polyurethane thinned 50/50 with thinner. I let it dry and went back to rub the first coat down with 0000 steel wool when I noticed a problem.
On various spots of the walnut there is a white discoloration. It is under the first coat of poly and looks like either trapped air or wax. But I haven't even had wax near the pieces! The discolored areas tend to be on corners or edges but on one piece it looks like there are white dots in the grain, scattered over a flat surface.
I resanded one piece down to the wood and was able to make it go away but matching the finish after getting that far is not all that easy. And it doesn't tell me what the problem was.
My initial thought was trapped dust and that may be what it is, but I brushed and rubbed the heck out of these pieces before going to the poly. It has been running in the low 90's with about 25% humidity (super dry) here in Colorado over the last week and the poly has been drying very fast. I don't know if that has anything to do with this problem.
Any thoughts or suggestions? If we don't get a quick answer I'll post some pictures to further illustrate the issue. Thanks ahead for helping out!
I would guess You have you either oil push back(bubbling up from the wood) or trapped moister. I think your only recourse is to sand it back and re-coat, but you should give a 1lb cut coat of shellac first be fore applying finish again. It would be a good idea to do a test on another piece of walnut just in case your finish has gone bad.
All of the finishes are oil based so I think we can eliminate the bad combination. I think that either I rushed the application of the poly or there might have been oil coming up from the wood. With the temperature as high as it was I may have thought it was dryer than it really was and moved on to the next step too early.
I'm not a big shop so the finish going bad is always a possibility. I'll also try not cutting the poly so much. I didn't realize that it could have that effect if it was mixed too thin.
I appreciate the time and opinions. If there are more, I'm listening.
before you strip the finish off…scuff sand it with some fine paper 320 or finer, and apply wipe a small area with more finish, this can occur sometimes with as jim stated the oil pushing back into the top coat…minwax is BLO , and the solvents in the poly can redissolve it and it mixes some what…if after scuffing and a light wipe of more finish it goes away , all is well, if not then sand it down further…and dont thin the poly…poly is a film finish and thinning it by 50% is too much, 25 at the outside….if the poly is a satin or semi gloss , youmay be getting the flattening agent thats used to reduce the sheen, if it speperates from the finish it will appear white…the areas you have described are fast absorbtion areas, which can suck the finish in and leave the flattening agent as well they can appear white when in reality its the absence of finish from quick absorbtion…again try light scuffing and another coat…it shoulod take care of it…
I always wipe down with lacquer thinner before applying stain, or finish. That's to insure a clean surface. Not saying yours wasn't clean, though. It also seems to dry the wood of oils and moisture. We're pretty dry here in AZ, too. Can't imagine it's moisture. Maybe, as suggested earlier, too much thinner and too fast drying?
Good luck, and please keep us informed of your progress in fixing the problem.
I don't use poly unless the object is to produce a very hard finish. I use a wiping varnish with a gloss that can be rubbed to the desired sheen. Prep with BLO to "pop" the grain first. Poly is just too "plastic" for my tastes.
Barry-Excellent information and it fits like a glove. I'll give it a try. I guess I need to take my own advice and walk away from the piece for a while once I apply part of the finish.
Bill-I can understand the view that poly is too plastic. Given that most of my work is mant to be handled I have been erring on the side of durability. I think this might be a sign to explore other options. It'll help me grow a little as a woodworker as well.
durnik i don't know if your interested, but you did mention wanting to learn more on wood finishing their is a great book out their named Understanding Wood Finishing by Bob Flexner. i found this book to be indispensable bob used to teach at the college i was an instructor at and when it comes to finishing this guy knows everything. i personally lean twords lacquer for simplicity but it usually requires spray equipment.
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