I often get questions about the use of lifting straps, forearm training, and of course, "How do I build a barn-door back?" They're often separate questions, but all three are somewhat related.
Many coaches recommend limiting the use of lifting straps. I have mixed feelings about that. Sure, not using straps will increase the demands on grip strength and will also provide some stimulus for the forearms (grip is at least as much about hands as it is forearms).
But what if your goal is overall muscle growth? What if NOT using straps takes 50 pounds or more off of your deadlift sets? What's more impactful and important to you? Getting in some extra grip and forearm work, or giving yourself the ability to lift heavy weights for more reps?
Some people are also arm dominant. Their arms take over during back exercises. Using straps on pulling exercises allows them to better target their back.
So, if your MAIN goal is building up the forearms, limiting the use of straps is a good idea, but understand that straps aren't evil or "sissy" either!
As for forearm work, I like to use a thick bar for curls and reverse curls as well as Fat Gripz for hammer curls.
I don't use them for back or pulling exercises because they'll severely limit how much weight you can use, decreasing the stimulus on the main muscles you're trying to make bigger.
I also believe in constant tension when training the forearms. Maximizing lactic acid accumulation is the best way to make the forearms bigger. As such, I like the good old wrist roller.
That's been my go-to exercise for bigger forearms. Since there's basically no eccentric (begtaive) and no muscle damage going on, you can easily do them on a daily basis even multiple times per day.
I don't hate what some consider "sissy" forearm exercises, like wrist curls and wrist extensions, but most people do them wrong. The range of motion is super short so you must use constant tension and slow the movement if you want them to work. We want each set to last at least 40 seconds, or more precisely you want maximum lactic acid accumulation.
Finally, I've always been reluctant to use occlusion training (using a band or wrap to reduce blood flow to the working muscle) but I believe that it could be a great tool for forearm size. But I'd have to experiment with it first to be able to recommend it.