We get this question a lot from customers working on their own little DIY projects. No your brush is not ruined, as long as you buy some paint thinner and spend the time to completely and thoroughly clean the brush. The trouble is that the cost of thinner these days is so high that it’s really not worth it so save your brush afterwards for another project, especially since in order to make your brush truly clean and ready for another project, you must go through a lot of thinner and spend a lot of your valuable time. If you have used the brush for an entire day and the edges of the brush are crusty with dried stain/paint from all-day exposure, then forget it, throw it away.
Extra tip: If you want to save your brush for next-day usage, stick it into a baggie with about a tablespoon’s worth of the stain/paint you’re using and try to seal it as much as possible…a little exposure to air shouldn’t hurt it as long as you’re using it the following day or 2.