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This is probably most applicable for water-based finishes, but I suppose it could be generalized to all finishes.

Once a finish has dried / cured, do I need to worry about incidental contact between a finish and its thinner / solvent? I'm not talking about leaving a piece finished with water-based poly outside in the rain, of course, but rather near a shower or sink, for instance. Will that contact with water eventually eat away the finish? Would it depend on the type of finish and/or how many coats and/or how thick each coat was?

I doubt you'd have a similar situation with oil-based finishes outside the shop, but what if you spilled mineral spirits, for example? Would you need to worry about refinishing?

  • With the waterbased finishes specifically, as far as water is concerned the best of them won't be affected once completely dry (cured), being water-resistant is one of the things they're specifically designed to be. But quality does vary in finishes and a cheap one might not be as good as the finish can be. Re. varnish and mineral spirits, once fully cured the varnish is pretty inert and won't be affected by incidental contact, only prolonged exposure might begin to weaken and break it down. – Graphus May 2 '17 at 7:41
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The answer is, it depends. Finishes that "cure" after drying (like most oil-based products) will typically not be damaged by incidental contact with their solvent. Finishes that only "dry" but do not cure (like shellac) can be removed with their solvent more-or-less indefinitely.

In your question specifically, water-borne products are not water soluble after they've cured. Water-borne products are not actually water-based. They have a finish material that is soluble in some solvent (typically glycol ether I think) that is itself soluble in water. So they actually have 3 phases: water drying, solvent drying, finish curing. Once the solvent has flashed off the finish is not water soluble at all.

See the related question What is the difference between "curing" and "drying"? for more information

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