I never found one, but it's pretty intuitive.
From the 'layer' menu, select 'add 3D layer', choose a model* and start painting.
There's a tool hidden away under the eyedropper tool (click and hold to see the sub-items). The 3D Tool allows manipulation of the camera and the object while painting, as well as different lighting and rendering solutions (redundant, but interesting).
The standard flat-view bitmap can be accessed by double clicking the sub-maps of the layer, you can also add layers and artwork directly to this file, save it, and the results update on the 3d model in the other file. This is also the file you save-as-jpg from for your final skin file.
Freehand is a lot of fun (I drew that top skin with the mouse) and you can use multi-point marquee-selections across entire sections. eg, the lightning bolts - similar to your proposed spirals.
By far the best use for it is with transferring complex existing artwork to intersecting panels with perfect alignment. You can use the clone tool to transfer stickers and logos, etc. by placing them on a new layer and cloning them to the car/helmet model layer.
It has a few failings, especially in the older versions, but things like angular tearing, normal glitches and the like don't happen so often in the latest version. The intersections between the faces is a lot smoother now too, but some, as you can see from the side panels of the helmet, still need a bit of manual correction.
* this is a whole other thing... objs are more stable, but models and converter apps are impossible to find, 3ds files are achievable, but need a bit of tidying up to be usable - either way, it will pay to know a little bit about 3D modelling.