Painting an STL file I have an STL file that I want to add a tribal tattoo to with the end goal of getting it printed in colour sandstone from Shapeways, or in full colour on an Mcor Arke at some point, but I'm not really sure where to go. 

I've looked at Microsoft Paint 3D. It's ok, but I cannot get the paint job symmetric. It outputs the .3mf format which the Arke will eventually handle with their Orange software, but it won't export into anything Shapeways will accept.

I've looked at UV unwrapping in Blender, but it seems I have to create the image in another graphics program and then import it... but I can't really work out where the images will be applied to the object, and then when I'm done it seems I cannot export the bits I'm expecting. On the plus side I have the mirror function so I can just paint one side and it's symmetrical.

I've looked briefly at Maya and 3Ds Max, but they are eyewateringly expensive, way out of my league.

Blender with some form of layers for the image might be really useful so I can make the tattoo in bits and then apply them to parts. I've kind of seen how you can split the model into panels but my brain is struggling to get round this as it's really quite hit and miss... and exporting seems to be a problem, although DAE and the texture will go to Shapeways, assuming the DAE has the texture layout embedded. FBX seems to go into Paint 3D and then I can export it to 3MF for the Arke.

Has anyone got any helping pointers for this confused noob?

Thanks
It is indeed more related to the blender forum  than the 3d printing one. 

You can start by looking at this post from blender.stackexchange.com which explain how to unwrap and paint on a 3d object. You can also paint the texture on a 2d software like Gimp and then put it in the UV Image editor of Blender. Then you should be able to export your project (with the texture) in a suitable format for 3d print.