When you translate CAD data from one format to another there is often a loss of data integrity. This stems from the different ways modeling kernels treat the equations that drive surfaces. If you were to look at the guts of a CAD file you would find a description of a whole bunch of surfaces and how they are trimmed/joined. While the intent is that generic CAD files are able to be interpreted by every system, the reality is that the differences in kernels create occasional errors.
What you're seeing is one such error.
In order to save on computing requirements, 3D models employ some unique attributes. One of the attributes of the individual surfaces is their normal direction. Normal (just like in physics) refers to a perpendicularity. Normals save computing power because it lets the computer know that only one "side" of that surface is important. The reason you see brown patches on the model is because the normals have flipped to the inside for those areas. The red appearance is being shown on the inside of the chair in this case. Fusion is very user friendly in that it shows you these flipped normals (as brown). A lot of surfacing tools treat the back-side of a normal as totally invisible (which can be an absolute nightmare).
I haven't downloaded the file just yet but I wanted to throw this out there.
You might be able to fix the model just by entering the patch workspace and stitching everything back together. If the surfaces are properly stitched, the normal of the surface is no longer an issue (the kernel understands that a water-tight volume inherently has outward facing normals). You may need to reverse the normal on some of these to get good results, though.
The other thing is that typically when you see an error like flipped normals, you should be on the lookout for other surface errors - untrimmed surfaces, self-intersections, or holes left by different approximations. You'll notice when you stitch surfaces back together you are able to specify a tolerance - this is because there are gaps and/or approximations that may be present.
IGES files are the worst offenders for wonky surfaces like this. Often times you'll find filleted edges come out as full cylinders or toroids. Generally, I stick to STEP or SAT where possible.
K. Cornett
Generative Design Consultant / Trainer
