sand blasting slow

The machinist at my workplace has a sandblasting cabinet, somewhere between hobbyist and industrial. I thought it would be nice to clean up some old wheels, but it took me about 2 hours per wheel to get them clean, and even then they weren't as clean as you might expect. The picture shows what I got done in 10 minutes. It takes about 5 seconds to remove paint and primer from a pea-sized area. How can we get the blasting cabinet to work better?