I had this very badly when I was about 23-24. I used to work a couple hours a day on a buffing wheel... holding brass tubes hard against the wheel to grind off nicks. Over the period of about a year by favoring my right arm... I built it up an inch larger than my left.
The only way to fix this is to go to dumbell work, but you have to be very specific how you work the dumbbells:
Start with your weak arm FIRST.
If you can do say 8 reps with 50 lbs doing dumbbell concentration curls with your weak arm... then stop at 8 reps when you do the curls with your strong arm.
Always go to failure on an exercise with your weak arm - then stop at that number with your strong are even though you can do more reps.
If you do this... you will balance out quickly.