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So, for lack of a proper shop, I was applying a first coat of undiluted Varathane Ultimate Polyurethane (oil based) to a plywood box using my dining room table under a plastic drop cloth as a work bench. The room was about 75F. The family came home and complained about the smell, so the drop cloth and project got moved to the garage where the temperature was 60F. The next day, I snuck the project back inside and hid it in a closet, once again at 75F. Six days later, the finish is still tacky to the touch.

I'm going to wait until it gets warmer in the garage (several months from now) before attempting any more coats of polyurethane, but I am curious whether I have damaged the current coat such that it will never cure. ...and if this is the case, that the existing coat will never cure, how do I remove it before starting over?

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    I can understand why you're worried and TBH I am surprised since the temps you say aren't that low, but finish drying is just one of those things that takes as long as it takes. "I am curious whether I have damaged the current coat such that it will never cure" With waterbased coatings this is a thing apparently, but AFAIK not with oil-based. And my experience has been that no matter how low the temp — I've applied varnish in an unheated room during winter, so from 4-12°C (39-54°F!) — and it always, without a single exception, cured eventually.
    – Graphus
    Jan 19 at 6:15
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    If you did want to remove it regular mineral spirits should be enough, although the varnish won't immediately dissolve like when fresh (it's partly cured already). If it's too resistant to just normal MS then xylene or toluene are more powerful solvents in the same family. But be aware they're a lot more malodorous and you need to be a little more cautious of the vapours.
    – Graphus
    Jan 19 at 6:19

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