Blotchy after sanding

[HonketyHank]

I have a 100+ year old waldzither that has seen more than its share of abuse, yet it is quite playable (and sounds pretty good). I have been looking at the cowboy buckle rash and pick scars for over a year now and have always thought it could look pretty good if refinished - no major dents or gouges, most of the scrapes and scabs are in the finish.

A few days ago I started on the back, which I am pretty sure is rosewood. I sanded (with minimal pressure) with 150 grit paper until just barely through the finish, then got the remaining traces with 240. Everything looked nice with pretty even color (except for the grain, of course).

I quit for the day and came back the next day to begin on the sides and noticed that the back was blotchy. Some of the blotches even looking oily. See photo.



I have read that rosewood is 'resinous' and can be problematic with some kinds of finishes. Is that what I am seeing? My plan was to use a wiping varnish mixture - 50/50 semigloss polyurethane and mineral spirits. Maybe three or four coats. Is this going to be a problem? Do I need to treat or seal the rosewood prior to applying the finish? Any other advice or potential pitfalls to watch out for?

Thanks

[sunburst]

First of all, as usual on this site, I'm only looking at an internet-posted picture on a computer monitor, so I'm really only guessing here.
Looks like some sort of contaminate in the wood. Could be oil, some kind of "guitar polish", something applied to the instrument at some time,... who knows.
You might have to seal with shellac or vinyl sealer to isolate it from your finish. It looks like it is in rather than on the wood, so removing whatever it is may not be possible. You will have to choose your finish and then choose your sealer according to compatibility.

(If it is some sort of applied "polish" it might contain silicone and that would likely be the worst case scenario. Might have to use fish eye eliminator in the sealer and/or finish.)

[multidon]

I doubt it’s resin in the wood. If that was the case it would be everywhere. I am with John, I believe it is a contaminant. Have you tried naphtha? It’s a pretty good general degreaser. If that doesn’t work I would try shellac as a sealer and base coat. It sticks to just about anything.

[HonketyHank]

I just now resanded the upper half of the back (150, then 240 grit). Here's a shot. I will wait and see if the dark areas 'redevelop'. If they do, I'll go out to the shop later and try some naptha.



If it is some kind of contaminant, it has been there for quite a while. The finish was pretty much intact over that area near the neck joint - maybe a few scratches but no big areas of wood exposed. Those dark areas were not visible before I sanded off the finish.

[Marty Jacobson]

Cool waldzither!

[HonketyHank]

I think I found a simple answer.

I just looked everything over after about 20 hrs from when last picture was taken and I can see no difference. I suspect the blotches are/were just areas where I had not quite sanded through the original finish. I suspect maybe because for some reason the original finish penetrated a bit more into the wood.

So I plan to sand just a bit more and see if everything evens out better.