Hmmmm....wasn't there just a long thread on the forearms?
Anyway, the thumbs are also important in forearm/grip work, so exercises like pinch gripping a 2X4 and doing chins, pinch grip a pair of smooth 45's, sandbag training, etc can be included. Rope climbing (excellent for the lats and biceps) is another movement worth mentioning.
The finger tips are also important, So might try doing those chins, on that 2X4, with just the finger tips. Either pinch grip or finger tips, try hanging from that 2X4 (building up to holding a DB in the free hand), even slight to quarter chins are rewarding.. Either grip (pinch/finger tip) tends to bring our those cord like muscles/tendons in the forearms....can be very impressive.
Been around some rock climbers a while back. Swear, some of the more impressive forearms around. Almost have to believe some of the forearms measured more that the upper arms. Popeye, indeed. Good Luck.