As someone who uses polyester resin and glass fibre mat regularly in my designs, I have found that acetone is the most expensive way of cleaning brushes and rollers before they harden. Acetone can be used only twice before the brushes are
no longer cleaned properly, after which they go hard.
You may also use the following as a post-cleaning boost if old and dirty acetone has been used, or to clean shorthair rollers: dip and work the brush well in a small pool of Handy Andy or good-quality dishwashing liquid. Wash the brush under a tap; repeat if the brush feels sticky. Make sure the brush is completely dry before using it on another batch of resin, as any water in resin halts the chemical hardening process. Warning: This works only if the polyester resin is not in the final stage of setting. -HUGH ROBINSON, DURBAN