We all use rF as our viewer. You save to DDS as you go and load it in game so you can view it in the showroom.
A shame rF2 is such crap even for such a simple task rF does in seconds.
Just make sure you save the PSD at intervals so you keep a layered version.
There is a way you can paint 3D now. But you need the 3D Max mesh file and load it into Photoshop. Doesn't work with GMT files though and no mod group are going to hand out their 3D Max files. Just use rFactor.