I had the same problem. I used to just end up fatiguing my forearms (and joints) trying different curls to no avail, feeling the barest stimulation in the biceps.
I found one simple trick that seemed to help. Hard to explain, but when curling dumbells, I contract at the end towards my side delts slightly more than usual.
I then hammer curl like a motherfucker and can’t drink from a cup all day…
Yes, my forearms burn out first as well. Tried about every variation of grip, but still can’t isolate the work like I can with other muscle groups. Started doing the farmer’s walk with 100lb db’s (3 sets of about 20 paces down and 20 paces back at the end of my workout) to strengthen my grip and forearms.
Try these 3 things when you next curl.
In order to remove the forearms from the movement hold the DB’s/BB with your hands pointing behind you (the opposite to the concentric portion of a wrist curl)
If you are using higher reps add extra resistance yourself by pushing against the contraction with your biceps while they are contracting (it’s a little hard to explain but it’s how you make light weight heavy).
At the top of the curl (assuming you aren’t cheating and using you’re delts at this point) squeeze your biceps as though you are trying to burst them.
Let me know how you find it as that’s how I managed to break through a plateau I hit a while ago with my arms (I would get tired but I wasn’t actually breaking down the muscle or getting any stronger)