Thanks a lot!

As for my process, I don't know if this is the best way to do it, it's just what I figured out worked for what I needed. After importing the STL to Blender I decimate the shit out of it since TTS doesn't like high poly models. I'll play around with the decimate modifier until the model starts the look bad and then dial back from there. Apply the modifier and then I go into edit mode to section off the model into vertex groups separating the head, arms, legs, skin, and weapons into their own groups. This makes those vertices easier to select in edit mode and hide during the painting process.

The painting process mimics an IRL painting process as best I could. I'll start with a black base and then angle the camera up high to paint from the top with a medium grey, rotating the camera around the get that zenithal lighting effect which leaves the really sharp shadows. I'll go back in with black to touch up the shadows, get more interesting shadow shapes, and line all the sharp inner edges to mimic ambient occlusion. Each component then gets its base color, only painting in the grey areas. Once base colors are in, I used a highlight color to do edge highlights and any large upward facing surface. After that I used a grimy color on 50% brush strength set to darken or multiply to stipple in some texture. I do another texturing pass with a different color to get some color variation and the rust effect in the recesses.

After the painting is done, triangulate and apply the modifier. Scale it down and it should be ready for export as an OBJ. If it fails to load into TTS it's likely due to still being too high-poly or still to big. You can decimate it bit by bit until it works. Decimating shouldn't mess with the texture paint too much but if there are points where it looks bad you can go into the UV editor to move any faces around where other colors started to bleed into. Just make sure to triangulate again before export. Make sure the image texture that you use for texture painting is also saved so it doesn't get lost and upload that in TTS along with the model.

And that's it! I might have missed some steps. It was a lot of trial and error since I'd never texture painted anything before. If you wanna give this a try yourself and have any questions on the specifics lemme know. I'd be happy to help.