Not many bodybuilders develop 18 inch arms. We don't include obese individuals. Bulking up is imperative for hard gainers.
Doesn't even have to be obese.
Besides, people focusing on biceps to put size on their arms are spinning their wheels.
Muscular arm size for the sake of muscular arm size you're much better off focusing on the long head of the tricep with overhead extension movements that put the lateral head in a weak position to contract so the workload is shifted to the long head /run-on sentence. The Finnish off with a couple sets of 2 other angles. I'd prefer with the upper arm perpendicular to the torso as with a skull crusher. I prefer with an Oly straight bar, and maybe with the last set done with a couple KB's hanging from the ends of the bar secured with resistance bands. Maybe 2 sets, plus that last light set with plates plus the Kettlebells. Then Finnish with pushdowns with upper arms parallel or close to it for a couple sets of as many as you can get the stack for. 20, 30, 40 etc and flaring your elbows out and getting your bodyweight over the handle when you hit the wall doing them strict with elbows tucked into your sides. Couple sets should be fine. Maybe 3 with the final one being with your torso bent forward about 45 degrees and stepping back from the cable pulling over head and having your elbows out in front of your shoulders almost cell year torso and upper arms are in the same position as a skull crusher and pressing out instead of down. That way you kind of mimic the angle of a Skullcrusher except you have tension the entire range of motion whereas with the skull crusher you lose tension at the top of the movement.... But still, put all your focus primarily on the overhead extension stuff and Blasting the long head of the tricep which will have the greatest impact on arm size overall. And not to forget to blast the brachialis muscles as well with hammer curls, rope curls, and reverse grip curls both with this straight wrist and a broken wrist. I'd prefer to focus more on curling in that fashion first and finish up with proper form preacher curls. Hella considered a nuisance jump over fences and move out into fields are moving to brush you'll notice how this metal fences bit and boat right here it's usually because the nilgai which is a big animal me 400 pounds or more jumps over this fence and it will catch it in bend it is they go over there but the mail guy will jump over them especially when the dog gives Chase most trainers neglect the brachialis and lack that 3D pop The brachialis gives out to the side of the arm between the bicep and lateral head of the tricep