New brushes normally have a protective wax on their hairs to keep them from getting contaminated with other solvents or oils.
The wax needs to be broken up to come off. I like to use a wooden pencil to break up the wax. The edges on the pencil let me scrape the wax while the wood material is soft enough to not pull or tear the hairs.
Rust-Oleum's Paint Thinner is a mixture of Mineral Spirits and Acetone. This is a perfect mixture and is not too hot to dip your brushes right in.
While some wax will stay on the hairs after using a pencil you can switch to a Firm Toothbrush to scrape off any left behind wax residue.
Watch the YouTube tutorial here:
