imo, iso contraction (like people get from deads) is mediocre at best for forearm DEVELOPMENT (iow size vs. strength factors).
i've trained with enough pl'ers and OL'ers (the latter use hook grip, btw) to conclude that you can build very very strong grip strength while getting concommitantly mediocre DEVELOPMENT.
moving the muscles through a greater ROM, for higher reps than the 1,2,3's that strength athletes usually do, will yield superior visual results, imo
lots of exercises you can do, including the classic rolling up a weight hanging from a rope (thicker bar yields better results), doing wrist raises with a plate, etc.
some of the best forearms i have ever seen have been on merchant marines. completely popeye like (which is where the cartoonist got his ideas - sailors who work with ropes for hours on end).
MUCH longer time under tension, and much greater rom than one will get with deadlifts and the like. and much more impressive forearms.