Can't access some polygons to fill... what am I doing wrong?

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So, I am new to modeling and texturing. I am building some starships using blender (using parts from Shipyard) and trying to texture them. I'm... learning. I built the models by pasting together existing pieces, almost always using the mirror modifier in Blender.

Well, as I am trying to work with them in Substance Painter, I notice that wherever I have a part that is doubled, one of them is just fine, but the other seems see-through. Like, there are red ploygons, but they are transparent.  I can't access its polygons. It looks normal in I-ray, though. But, if I try to texture it, it's like the textures show up on the inside, rather than the outside.

An example is this model I am working on (attached). The lower "warp engine" appears to have glow only on the inside. The walls are see through, whereas the ones on the top are solid. When I look at in in I-ray, the one on top looks right, but the one on bottom is just basic textures. Anything I have masked out to be emissive doesn't show. What am I doing wrong?

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Here's a closer shot of the lower engine.

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Vs. the upper one.

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And in Iray.

[CAGameDEV]

Show your UVs. If you're dealing with mirrored parts you should decide if they will hold the same texture information or not. If so, just offset them to the next UV space and work on that single side, otherwise make sure you apply that modifier and don't have doubles or overlapping in your UV map.

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Thank you! That's exactly what I need to know. I am new enough at this that I'm not exactly sure how to do what you are saying, though. Could anyone help with instructions on how to:
1. Show my UVs.
2. Apply the modifiers as suggested.
3. Offset something to the next UV space.

Thanks again. I do apologize... I am terribly new at this!

[CAGameDEV]

Thank you! That's exactly what I need to know. I am new enough at this that I'm not exactly sure how to do what you are saying, though. Could anyone help with instructions on how to:
1. Show my UVs.
2. Apply the modifiers as suggested.
3. Offset something to the next UV space.

Thanks again. I do apologize... I am terribly new at this!

Are you kit-bashing by chance? If so, I'm assuming all the UVs are pre-set per part?

In Blender 2.8 go into your UV screen, select the parts and go into Edit mode, your UVs will show:



When you export your model as an FBX you'll see the option, or you can just duplicate your object and apply the modifiers then export it out.

To offset just select the UV island and hit G then 1.

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I am one part kit bashing, one part making my own parts. This is really helpful! So, I will try that. It sounds like it needs to be FBX format, not OBJ format. That may have been where I ran into trouble.

[CAGameDEV]

You can use OBJs, I just don't know if they auto apply modifiers or not... Either way it's best to duplicate and apply then export. Always keep a non-destructive version if you're using Blender's modifiers on a mesh.