"My advice would be that if you're adding a hardener, stick to the hardener that the paint mfg recommends."

Truer words were never spoken. Every pro painter that I have ever spoken with starts any discussion about using any add-in chemicals with "only use what is recommended by the paint manufacturer".

You are dealing with a chemical reaction (not just evaporation of solvents) and need to be certain that all combinations of chemicals work correctly together. Stick with the paint system's products. As mentioned above, therer is a Valspar hardener available for Valspar BPS paints.