Training the fingers is a process that uses a bucket of medium that your hands are not struck into, you use slow smooth strength and see how far you can push your entire arm from the shoulders down into the bucket. This will be hard at first as your flexor and extender muscles of the lower arm/forearm are not strong and you will notice you cannot get all the way through the medium.
Most people start with beans. Does NOT matter what kind so please do not come back with all that silly superstition about you NEED mung beans. Many people like soy beans as they are very hard and round. You could use kidney beans, etc.
You will practice pushing your arm(hand, forearm, upper arm) into the medium.
You then practice the various flapping methods with your hands under the beans. You also twirl the hands as if opening a door knob and squeezing the beans, you then can add grabbing the beans and then flicking them back into the bucket with your finger flicks. Many South Mantis people train their hands and fingers this way. They have nasty 3 hand power.
You need to use some strong dit da jow as the finger and forearms are going to get sore as heck with all this supination and pronation of the arms. Before and after are the usual applications methods.
You can also get yourself some solid Bao Ding Wan(
http://www.baodingballs.com) as the hollow ones are for cupcakes.
Working the solid bao ding with the fingers will increase their strength as well.
All together the flapping, twirling exercises along with pushing through 50 pounds of beans in a bucket is strengthening the arms in a way that forearm curls, and weight lifting does not.
Add this to Iron Palm training and you are going to have a unusually strong set of arms, hands, along with a grip strength increase.
Other Shifu teach you to pick up jars that are heavy with the fingertips, this is practiced in many of the Okinawan systems as well. Make the mitts strong and then rip people in half when needed.
Hope this helps.