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.
Best Answer
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.
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.