Tack Rag Residue on Spar Varnish

      Cautionary tale: wetting a tack rag with mineral spirits before use may deposit "tack" onto your workpiece. April 19, 2011

We have a set of interior shutters that were stained and then sprayed with one coat of spar varnish. We lightly sanded the panels, getting ready for a second and final coat of varnish. We typically tack rag them at this point, but a new guy in our shop dampened (wet) the rags with mineral spirits (paint thinner) and wiped them down. The next morning they were still tacky, almost like the varnish had softened. We put fans on the panels, and five days later there is no change - still sticky. I am starting to believe this is just residue from the wet tack rags, but was wondering if we could wipe them with something else prior to respraying. I am hesitant to respray with them in this condition.

Forum Responses
(Finishing Forum)
From the original questioner:
After my first post, I wiped the back of one panel down with lacquer thinner. After drying, the sticky residue was mostly gone, so I believe there was no harm done to the varnish itself. I did not want to believe that a light wiping of paint thinner would soften the varnish, and now I am more convinced.

I have 10 panels on this order, all of them operable louver. Is there a safer (for the finish) or easier way to remove the residue without having compatibility issues when I spray the final coat?

From contributor G:
At a guess, I'd say the mineral spirits dissolved the tack out of the tack rags and left it on the shutter. You need to find the mildest solvent that will wash it off. Note the word wash. A fast wipe likely won't do it. Try soap and water first, then alcohol, then acetone.

From the original questioner:
Thanks. The lacquer thinner dries fast, which I like, so it does not dissolve the varnish. But it is leaving too much of the residue, which means applying it repeatedly. I had not thought of soap and water. Do you think that may cause a problem with my second coat?

From contributor L:
This seems like a good place to remind waterborne users to never use tack rags. What they leave behind will cause adhesion problems for waterborne finishes.

From contributor G:
Provided the soap (not detergent) idea works, if all the soap residue is rinsed off, you should be good. After the rinse, they can be wiped dry and then sanded for the second clear coat. This should deal with any lingering residue.

About tack rags - you can make your own with a bit of thinned varnish on a lint-free cloth. For waterborne tack rags, I've wet a lint-free cloth with isopropanol and water.

From the original questioner:
Thanks for the tip about not using tack rags on waterborne finishes. Did not know that. I have found out that wiping the panels with mineral spirits is removing most of the residue. Soap and water did not seem to do much. After the wipe down with the paint thinner we will lightly sand and tack again with a drier tack rag. Respray and expect the best of results. Thank you for the helpful suggestions.

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