This is a messy way to go. You need a thinner bucket. I don't understand why commercial painters love to fling thinner around, it's a toxin and a fire hazard. Anything you aerosolize, you breathe.
Not being snarky - being concerned for your health. And your pocket book. I get that.
I was trained to clean those brushes after the day by wiping them out, giving a good rinse in a thinner bucket to knock off any solids, and shampooing with Murphy's. Every Day After Painting. There's no reason for a brush to get to this state. The next morning, it's dry and clean and ready to go. You can also knock off any cruddy build up through the day in a thinner bucket. Which is a zinc or steel bucket, with a lid, with a grate inside to run the bristles over. Paint solids fall to the bottom and the thinner can be used until it's exhausted. No breathing micro droplets, none on the skin, and the fire hazard stays contained. Yes, paint thinner is a fire hazard. It comes from the same cracking tower as motor oil and gasoline, it's very dirty stuff. It often contains lead and other nasty things too. You don't want to breathe it.
Scrape off as much gunk as you can before soaking brush/brushes in hot vinegar. More tips on reuse and repair from us. This will work for paint or varnish. Scrape off as much gunk as you can. A quality paint brush cleaner will be able to remove paint, so you don’t have to worry about colors mixing in the future and keep your brushes in their best possible condition, conditioning the bristles to keep them looking their best. However, not all paint brush cleaners are created equal. When acrylic paint dries, it dries hard, into an almost plastic-like form. This can make it hard to restore a brush that has dried or get paint out of something like clothing. To clean acrylic paint from a brush, or surface, you must start by scraping any thick pieces off. “The most important thing to remember with regard to caring for your acrylic brushes is not to let the paint dry following use, as the paint is not soluble in this state and will produce a plastic-like dried texture.” - Winsor & Newton.
Paint Brush Dried Hardwood
You can also give most brushes a 72 hour soak in Murphys as a maintenance deep clean. Every six months or so. This can also rescue brushes you've already trashed. No scraping or grinding needed if you clean up every day though. That's time out of your life. Go watch a game! The five minutes to wash brushes is worth having a clean dry well conditioned tool at hand the next morning. But a Saturday afternoon? Nah.
Paint Brush Dried Hard Rock
I'm trained as a portrait painter, but I renovate houses too. And I have a brush fetish, I have over a thousand high quality ones. I clean them all the same way, studio or house. They're all in great shape. Washing daily is not a problem, the oil in oil soap is a great conditioner for natural hairs. Just let them air dry out in the open, they could mildew in a closed container. Takes them inside in freezing weather. Love them and they'll love you back.