charlie is right, belt dressing is for the older v belts, and even then they should be replaced.

the only thing i use belt dressing for in the past 10 years, spraying on the "open" sides of pulleys while the old belt is on there right before replacing the belt. Ive done this for two reasons: 1. an attempt at cleaning rust and such off the pullies bfore the new belt goes on. 2. as a cheap test for slippage to avoid a pricey /unavailable tesioner