How to fix WoW models in photon workshop

First of all, Fred's answer is very well but some more items to note, and a different way to fix them using blender.

After opening blender, deleting the box with entf and importing the Wavefront .obj, I changed to edit mode and started inspecting the colored areas for artifacts and what they were.

Layered surface only areas

The top feathers are made up from 3 layers of surfaces:

This could be easily fixed by removing the middle layer, then connecting the front and back with new faces. In investigating this I also noted, that many of the faces actually are unconnected, so we fix that first, running a Merge intersecting vertices: rightclick -> Merge Vertices -> By Distance. Then, all the lines are blue - this means, that the lines are determined as "sharp edges" so we change to edge select, A, then rightclick -> clear sharp. Much better. Now, select the vertices in between the front and back, and delete them via entf.

After that work-over, the area looks like this:

So, next is re-skinning those edges. Select the remaining edges on the outside in pairs, then F to create a face between them, rinse, and repeat. But that does not yet solve the facing issue, because the faces are an odd gradient, not pure color, and as is evident, all the normals (activate them via a menu):

All normals are messed up! so we need to recalculate them. Recalculating them is A till everything is selected, then Ctrl+Shift+N, and voila, that area is reworked to a good part:

Now, we are going to export the file as .stl: even though Cura can take .obj, it has trouble to analyze those for facing properly at times. So let's look if we fixed that area alright... and in Cura now looks like this (OBJ left, STL right):

However, as I will show later, this is the work-intensive way, so don't really bother with those areas, we'll fix them all at once later.

Single surface only areas

Next, the shoulder details. Those are also one surface only, so we need to give them some heft. I am talking these (grey) ones:

I hid the bulk of the model (the selected, orange parts) first using H so it is more visible. Then I selected the upper 'wing', then pressed E for extrude. Type in .01 to define that thickness as how much it should extrude it out. Repeat for the other part. ALT+H to unhide the parts we had hidden before and Voila:

However, the newly thickened parts did not intersect the body properly yet, so we need to fix that a little, as seen here:

So, select the area we need to fix and E for Extrude. Slowly pull till the two parts intersect neatly:

floating details

Next step: the floating gem in front of the head!

Here fixing gets tricky. We might either just delete the crystal and print it separately and fix it with a little piece of wire later, or we need to create a strut. So, let's do the latter! We select the edges on the backside of the crystal like so...

rightclick -> Subdivide and then in the lower left corner set it to 2:

Select the center and Extrude

Keep the area selected and G for moving it down into the head, till we are happy with the look:

superfluous surfaces

Then there are some areas that contain "extra" surfaces, those can be deleted, such as this rectangular one and this feather inside the arm:

Fixng most surface only areas

Another problematic area is the facemask, which is only a single layer, but wrapped around the face it is a little more tricky to extrude:

This area can be fixed by running a solidify operation, which also makes fixing the feathers individually obsolete! So, how to run solidify?

On the right bar, click the wrench, in the dropdown choose solidify, set the thickness to 0.01 and... voila! Mostly fixed... well, we need to fix a few normals, as you'll see here:

so, let's select everything with A, then recalculate normals with CTRL+SHIFT+N, make sure that inside is not checked in the lower right corner, so the modeled surface is on the outside shell... and we are DONE! Export as STL or OBJ and go to test it!

Check in Cura

Now, to check if we still have reversed surfaces, I imported into Acura and... all the red areas (overhangs) are where they belong: on the underside! So... yep, we are done. Make sure to have the unify intersecting shells option on. You also might want to add some more struts for floating shoulder armor and such... It's a labor intensive work, but in the end you'll have a printable object.