- Wrap with plastic bag or glove turned inside out
- Into the trash it goes.
Seriously. This is my SOP when brushing 2-part epoxies and LPUs, which are very difficult to clean out of brushes (even if a gallon of reducer didn't cost $80 USD). Don't spend $1.01 of solvent to clean a $1 brush.
Cheapie brushes are called "chip brushes". They make upscale versions, such as the RedTree Fooler, that feel almost like a real brush, but are priced to toss.
First, a couple of prep notes.
It goes without saying that you wear gloves. If you refuse to wear gloves and poison yourself, that's all on you. Keep in mind developing country paints often contain some very toxic metals, because their oxides make very durable pigments.
Latex=emulsion paints use synthetic brushes, not bristle. Everything else needs a natural bristle brush. Don't try painting oil=alkyd paints with synthetic brushes, but you can use common rollers.
On that tin of "white spirit", you know what, I don't care what that is. Because for cleaning brushes, really any compatible liquid will do the job, even gasoline, as long as it evaporates fully and doesn't leave residue that would contaminate the paint. We'll just call it a solvent. That need not be the same as what you use to thin the paint with, which should be a recommended chemical for that purpose; its proper name is diluent.
Your sense of reusing the cleaning solvent is good. However, the residue at the bottom of the can will contaminate future brushing efforts. After it has settled for several days, very carefully pour off the clear solvent on top into a new container. Then promptly and with gloves, use a paper towel to wipe out the unrecoverable solvent and settled paint solids out of the jar. The solids go into the trash. The jar goes in recycling, unless you have a jar shortage, then clean it perfect and reuse it.
Jars and cans are an essential supply. Collect them. Any jars used should be thoroughly cleaned of food residue with soap, rinsed and dried first. Best to do this ahead so nature can dry them perfectly for you. Sterility matters.
If you will be resuming painting the color within 48 hours, don't clean the brush. Wet the brush and roller with diluent, then tightly wrap it in plastic - triple bagged with excess air squeezed out. Store it in a cool place, it should be fine.
So, your brush.
Have 1-2 square metres (10-20 sq.ft.) of surfaces you don't mind painting. I use old election signs of my honorable opponent. Brush some of this surface until the brush is exhausted.
Re-whet the brush by dipping its tip into the old solvent that is contaminated with paint (which has settled) if you have any, otherwise clean solvent will do.
Repeat steps 1 and 2 until it seems like you're mostly painting clean-ish solvent.
Dip again into old, paint-contaminated solvent and now start wringing the brush out into paper towels or rags. Choose a color of rag that will let you see the paint color i.e. Not black rags on black paint. Also remember you are wearing gloves.
Repeat 4 between 0 and 2 times, until it feels like what's coming into the paper towels is about as clean as the contaminated solvent you're dipping in.
Now we can go a couple of ways.
The cool way
Pour the smallest possible jar of clean solvent about 1/2" (12mm) deep. Suspend the brush so its bristles are in the solvent, but suspended about 1/4" (6mm) above. Very important! The paint residue will wick off the brush and into the solvent,
Many green bonus points if you can fully cap off the container to prevent evaporation of the solvent.
Leave it this way for 24 hours. Then gently extract the brush without stirring up the solvent or letting the brush touch bottom. Shake it out vigorously, or spin it. Hang it to let it dry, and you should be all set.
Do not leave it like this for significantly longer than 24 hours or it will make a big mess.
The more tedious way
Pour a tiny bit (1/8-1/4", 3-6mm) of clean solvent into a clean jar.
Now repeat step 4, trying not to let the brush leak too much paint into this clean solvent, i.e. pull it out the moment it starts extruding color into the solvent. You can instead pour a small splash of this clean solvent onto the bristles, but if you are dripping any on the ground, you are using too much. Hopefully working out of a small container will help you control it better.
As the stuff coming off the brush gets cleaner, be more aggressive about removing solvent. Start shaking it vigorously or spinning it. Don't splatter the side of your house (or someones car!)
If the solvent in the jar seems too contaminated to do a good job, transfer it to the dirty-solvent jar, and pour some more clean. Often you don't even need to wipe out the jar!
Once you've finished either method, hang the brush. The handle has a hole for a reason. Either tie a string through and suspend it, or stick it on a finishing nail or peg in your pegboard. Don't let it lay flush against the wall.
The final test
Once the brush is fully dry, give the bristles a swish. They will be petrified solid, but they should easily snap out of it and then feel like a normal brush. If it takes effort to unstick the bristles, or if stuff falls out from betwee them, the brush is done. Into the trash it goes.
That used, well-cleaned brush is valuable. It has already done most of the bristle shedding it is going to do. That makes it a good choice for high-visibility finish coats where bristle shedding is a real nuisance. So I advise after 2 uses and successful cleanings, hoard the brush for that purpose!
Back in the old days, painters had precious, well-made brushes often made of badger, that they took loving care of.