When you have allowed your paint brushes to harden with paint on them, you can easily soften them by placing them in hot vinegar until paint loosens and the wash them in hot soapy water.
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Hardened paint brushes can often be made as good as new by simmering them in boiling vinegar for a few minutes, then washing them in soapy water.
I use Murphy Oil Soap, to clean my paint brushes. I do decoupage, and the brushes get gluey. So I will soak them overnight in the oil soap, and they are clean again.
Here is the absolute best paint brush cleaner recipe known to man. I take a sherbet-sized plastic container filled 2/3 with water and I add 2-3 drops dishwashing liquid.
I did not want to throw my $1.00 brush away. I am a bit thrifty. It has many other uses to come. I soaked in vinegar water for a day and the brush cleaned up, no more dried floor patch.
Use a laundry bottle. Cut the handle at the top just before it curves to join the bottle. You should have a straight tube that connects to the bottle at the bottom of the handle.
After cleaning your paintbrush, wrap the bristles tightly in a dry paper towel folded in half. The towel will get damp so you can shape the bristles in their original shape.
Vegetable oil cleans paint brushes and keeps them soft and pliable.
Don't mess around with cleaning brushes and paint tools inside your home. Take them directly outside, being careful not to drip inside the house, and clean them on your lawn with a hose.
The best course of action for maintaining your paint brushes is to clean them thoroughly after every use. But there are times when this does not happen. Depending on the type of paint used there are ways to revive a stiff brush and remove the old paint.