View your skp with hidden geometry 'on' and in mono-chrome mode in a style where back faces have a distinct color compared to the front.
Then you can see if there are any reversed faces - reverse them to match the rest [I recommend that you make a shortcut key to 'reverse faces' for speed].
Now view it in texture-mode, you might find some faces aren't colored - fix those...
You might also have some unneeded hidden.smoothed edges - erase those if the faces stay OK...
Only now export etc...
