There is an exercise device that most are not aware of. It is called the Rolling Thunder (google it) It is basically a very thick bar that revolves and weight is attached via a loading pin that is clipped to the bar. The best way to train is by doing timed holds, I would suggest varying the time using different weights. For example at your heaviest, hold for 5 sec. Reduce the weight and hold for 15 seconds, etc. I would think holding for 1 minute would be the longest. Though even longer could prove beneficial as well, experiment!
My last workout I worked up to as heavy as I could hold for a split second with each hand then reduced the weight , doing holds of 10, 20, 40 and 55 seconds. My forearms were pumped HUGE. Be sure to try this approach to improve your grip and forearms. It works!