The iPad is an obvious device for painters to try to use, though it does lack the pressure sensitivity of things like a Wacom tablet. But a finger is just not the same as a brush.
A number of companies are now offering brushes for the iPad that you just might find easier to use.
Joystickers offer the Flow, a paintbrush that your iPad can work with.
Nomad Brush have the Nomad Brush, another paintbrush that works on the iPad. This is a longer haired brush for more flowing action.
PenGo don’t have a brush but do have a pen stylus that could also be a good option for painters.
I know people who are loving painting on their iPads. While it is not my cup of tea personally, they are producing amazing work and if it rocks your boat then go for it.