The easiest way to work around this is to schedule regular brush cleanings within your painting schedule. This prevents excessive build-up. It is important to ensure that the water runs clean when cleaning to ensure that the paint has been properly removed. If you are using a jar and not running it under the tap, try to keep multiple water jars on hand and replace used water to keep your brushes clean. It may seem tedious, but it needs to be done in order to preserve your brushes.