deads and farmers only work your grip to the max if you use a double over hand grip, and a weight that can roll. Otherwise, you can use the cheater grip, and rely on your skin to hold the weight.
So...for grip
dead lifts with double over hand grip
farmers walk with DB's (not the farmer's implement)
curls, farmer's, deadlift with "fat" bar
My personal favorite is to buy a good gripper...like the "Heavy Grip"...these are grippers with up to 350 pounds to resist.
I also like picking up my bathroom scale, and seeing how much weight I can squeeze. Do sets of 5, holding for 5 to 10 seconds each. Set a personal best, and meet or beat it every time.
I don't do them, but hammer curls seem to be almost all forearms for me...