Well, your grip strength may improve -- unless you're using straps -- but you certainly won't get the vein-popping forearms of Ronnie Coleman if you don't put some serious, specific effort into them.
Get On The Fast-Track
Fortunately, it is neither very hard nor time-consuming to put your forearms on the fast-track to growth. First off, you have to recognize that your forearms are muscles like any other. That means they have to be hit hard, and then get ample time to rest and recover in order for growth to occur.
Training biceps one day, back the next, and then doing forearms on the third day is not the best option. Instead, train forearms together with biceps and avoid training back the day before or after the bicep/forearm workout, thus conserving energy and/or allowing recuperation. Personally, I try to keep a space at least 2-3 days to make sure I'm not short-changing myself.
Now that you know where the forearms belong in your workout schedule, it's time to get down to business. Since the forearms have a number of small, separate muscles, you have to alternate exercises to hit them all. However, like I said earlier, this is not necessarily time-consuming.
I have found that forearms are a typical "more is less" type of muscle group, where a few intense sets for each part can do wonders. In fact, you can make excellent gains with only 3 exercises per workout;
One in which you curl a weight with your knuckles pointing up (squarely hitting brachioradialis and the dorsalflexative muscles), one with your knuckles pointing to the sides (hitting bicep brachii and brachioradialis) and one in which you hold a weight and flex your wrist so that the palm approaches your forearm (hitting the palmarflexative muscles).
A good example of the first exercise is the reverse-grip bicep curl.
1. Stand straight with feet shoulder-width apart, slightly bent knees, abs tense and shoulders relaxed.
2. Grab a straight barbell with an overhand grip (thumbs down) with your hands about 6-8 inches apart and let it hang.
3. From the starting position, curl up while keeping your elbows snugly against your sides.
4. For extra intensity, pull your knuckles back as you curl up so that the top of your forearms gets a good squeeze.
5. Hold and flex for a second at the top, then slowly return to the starting position.
Variation: For variation, you can try doing this with a Scott board (normally used for bicep curls). Yet another option, if your gym is equipped with one, is that always-available machine where you roll a bar that is hooked to a small weight stack.
I have yet to see a distinct name for these machines, but if your gym has one, you know what I'm talking about. You simply grab the bar and flex your forearms in either direction, and that's pretty much all there's to it.
The one caveat is that you must actually use your forearms -- it is very easy to start a pushing/pulling motion that originates in your shoulders, so make a conscious effort to relax the rest of your body.
As for the second exercise, nothing beats the good old hammer curl!
1. Sit on a bench with the back angled about 45 degrees. Keep your feet securely planted on the ground.
2. Grab a pair of dumbbells, lean back against the pad and let your arms hang.
3. Keeping your thumbs up, curl the dumbbells up as far you can. Keep your shoulders relaxed.
4. Squeeze at the top for a second. For more focus on brachioradialis, you can tilt the dumbbells a little (pinkies moving outwards.)
5. Slowly let the dumbbells sink down until you feel a good stretch and repeat.
Comments: Many people like to alternate hands, allowing them to squirm from side to side and cheat. Sure, you'll handle bigger weight, but you're also robbing yourself of some of the benefit and increasing the risk of injury. Instead, synchronize your arms so that you curl both dumbbells at the same time.
Variation: There are many variations to this exercise. I prefer the seated version as it gives me a fuller range of motion and a good stretch at the bottom. However, there is nothing that prevents you from doing it standing, but make sure you're not swaying. Stand with your back against a wall if you tend to 'swing' the weights.
If you're standing, you can use a cable machine with a thick rope instead. Simply attach the rope to the lower pulley, grab the rope and curl away as you would with dumbbells. The benefit of this is more even resistance, as is always the case with pulleys vs. free weights. Last but not least, I've had one client who swore by the double-handed approach.
This variation calls for a single, very heavy dumbbell, which as the name implies is held with both hands. The range of motion is somewhat limited since you have to start out with slightly bent elbows, but this guy built pretty massive forearms using this method so I'm not to say this method doesn't work.
Last but not least, time for some forearm rolls. Done right, these will kill off whatever grip strength you may have left so these are natural finishers before hitting the shower.
1. Set the stops on a power-cage so that you can touch the barbell with your fingertips as it rests against the stops.
2. Stand straight with feet shoulder-width apart, slightly bent knees, abs tense and shoulders relaxed with your back against the power-cage.
3. Grab the barbell by bending your knees, NOT rounding your back.
4. Open your fists half-way and let the barbell roll down about an inch, but maintain control at all times.
5. Close your fists again and curl up your palm towards the back of your forearms as far as you can.
6. Squeeze for 2-3 seconds, then slowly return to the starting point.
Comments: Make sure to stay close to the power-cage (or better yet, inside it) so that the stops catch the bar if you lose your grip. This is easy to do as you get tired and might cramp up.
Gloves are a sticky issue. While gloves, especially the neoprene kind, improve the grip, they also limit the range of motion. A bare hand can close its grip further, but may also lose the grip if it is sweaty.
Variation: You can do this exercise with a cable machine, but there's really not that much difference due to the short, linear motion of the free weight option. However, if you prefer to do this one-handed, definitely go with the cable machine.