Today I got distracted while applying some Minwax Pro Spar Urethane (which apparently is water-based) and left the brush sitting on top the can. The distraction lead to a trip to lowes and the infamous hobby lobby. So of course the Poly was pretty well on it's way to being hard. Mineral spirits did nothing after sitting for about 30 mins and now it's sitting in warm water. I would really appreciate some suggestions.

So far everything I've found on this either relates to oil-based poly, cleaning practices immediately after use, or stripping from surfaces.

It's a synthetic brush, one of the Purdy XLs I think. Either way, trying not to spend money on more brushes at the moment. There's too many other projects going on around the hose that could use the $12-15.

  • The back of the can typically says something to the effect of "Dried product may be removed with Xxxx" or, worst case, "Dried product must be mechanically removed". Have you read it in full?
    – JPhi1618
    Sep 26, 2019 at 15:15

3 Answers 3


Soak it good and plenty in soapy water. Water based paints are pretty soft stuff, and 1/2 hour to dry isn't really enough to start them curing. Water should soften them up. Don't use hot water as that will accelerate cure.

You can try some fairly wimpy paint stripper if you have some hardware store brand around. You don't want the industrial tier strippers designed to remove alkyds and LPUs, they'd probably just melt the brush. Latex paint is fairly fragile stuff after only an hour to cure. I'm not concerned about the drying out, it's the curing that makes it irremovable from the brush.

The mineral spirits was a mistake. I can't tell whether it sunk you, by reacting adversely with the paint, or just preserved things until you got it in water. It was definitely the wrong thing. Be prepared to write off the brush as a learning experience.

Know your paint

Where this went wrong is unfamiliarity with the products you are painting. What type of paint is basic knowledge you need to know.

Right off the bat, you need to know what the paint's reducer (or diluent) is. That's the chemical you use to make the paint less thick. (reduces the thickness of the paint, or dilutes it). You need that because it'll thicken up due to evaporation. The reducer also tells you what chemical to be using for wipedown, brush cleanup, spill cleanup, etc.

  • Latex=water
  • alkyds="paint thinner"
  • 2-pack epoxies and LPUs=their proprietary chemical

For cost or environmental reasons, you may prefer to use some other solvent for auxiliary tasks like wipedown, brush cleanup, etc. However, it should still be quite similar (in the family) of the solvent. Reduce alkyd paint with proper paint thinner, but it can be cleaned up with gasoline or that "eco-green low-VOC" paint thinner substitute (labeled 'do not use to thin paint', really!)

Only the right solvent will do

Whatever your correct reducer and solvent is, something else is wrong. They are incompatible with each other.

So read the label on the product you are applying, or if it's too small get the PDF of the instructions off the Web. It will tell you what to use for a reducer and for cleanup.

So then, when you have that situation, you already know what the correct reducer is, so you just pour 1/8" deep into a cup, and set the brush in it. The reducer will wick up into the brush and keep it moist.

Also, you need to select brushes based on solvent

Latex uses a synthetic brush like that one. Oil, alkyd, and the 2-pack paints use natural bristle brushes.

Natural bristle brushes will waterlog with water-based products. Synthetic brushes might work for alkyds but will melt with 2-pack paints.

With alkyds, there's a real question whether it's even worth trying to clean up the brush. With 2-pack paints, there's no question at all; into the trash it goes.

There's something to be said for cheap brushes

As a heavy user of 2-pack paints, I go through a lot of brushes. I use an upscale version of the cheapie dollar store chip brush, so I'm paying maybe $1.30 a unit for a respectable bristle brush. I try to clean it if I've used it for alkyds, because a once-used (broken in) brush is ideal for a 2-pack topcoat.

For latex paints, if you're the sort to abuse brushes, just buy less expensive brushes. On the other hand, there's no excuse for abusing a synthetic brush, since they are so easy to clean.

  • Well, I used a synthetic brush because the can stated synthetic, and I assumed that once an outdoor product dries, it should be largely water repellent, and I went with mineral spirits because I couldn't find an answer involving polyurethane that didn't involve mineral spirits. Not even on the can regarding dried product. Obviously those answers were mainly geared toward oil-based product but I went with what I had. The water helped, and the brush is largely back. There are a few bits at the tip left. Thank you for the depth of your answer though. You're the only one who went with water. Sep 26, 2019 at 17:05

Acetone is supposedly safe on nylon and polyester (my purdy xl brush states that it's a blend), and may possibly be effective (it tends to at least soften oil based poly, but I've never used it on water based poly). I've never had to clean anything off it, so I'm honestly not sure.

If acetone fails, at least $15 is a relatively cheap mistake to make. Most other solvents WILL damage the brush.


My experience is that once you have left the brush to dry, either partially or fully, the poly will have chemically cured to the point that it will not be cleanable by any tame cleaning method.

The only hope at all is that possibly some of the strongest paint and finish strippers could soften and remove the poly in the brush. Success is unlikely though simply due to the big wad of the poly in the brush. The finish stripper is made to work on removing the thin coat on a surface that is at best a few 1/100ths of an inch thick. It would take a lot of stripper to completely clean the brush...if it did not in the mean time start to destroy the brush.

In the end you will have spent hours trying to clean the brush and may very well have spent two or three times the cost of the brush in the cost off stripper.

The lesson here is that if you value your brushes clean them promptly and so not chase off on an errand without taking care of the brushes first.


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