You may have uncovered slightly deteriorated wood portions from your thorough sanding. Also, you may have used a rancid or old lacquer, or used a brush that wasn't thoroughly cleaned from a previous use. How to fix it would depend on the cause of the problem or how extensive it is. You could continue to sand to get all of the discoloration and putty, but the results may vary. For improper use of a tainted brush or bad lacquer, it's best to sand it again. But take it easy and/or use lacquer thinner if it's still fresh and remove the old varnish or lacquer. If it is extensive, you should consider the service of a professional, unless you have plenty of time and don't care if it comes out right or not.

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