Pools, Spas and Hot Tubs - Cannot remove impeller from motor shaft

I tried to replace my pump motor last night, but I could not get the impeller off the motor shaft. The impeller is nylon with a 1/2" molded nut on the front, back of the shaft has a slot for a screwdriver. What direction should I be turning? The shaft has some rust, what can I use to help release it? Thanks.

Those are usually left-hand threads, meaning you turn them clockwise to loosen them. If you know which way the motor turns when it's running, you need to turn the shaft in the opposite direction while holding the impeller. The shaft is designed such that the force of the shaft would tend to tighten the impeller.

Rust is probably not the problem. The impeller fits tightly on the shaft, as you've noticed, which prevents water from reaching the threads. When you finally get the impeller off, the threads will be the only clean, shiny part of the shaft.

If the slot on the back of the shaft is the screw that holds the rev limiter plate to the shaft, try removing it. There may be flat spots under there you can get a wrench on.