I ultimately ended up with this one (several suggested this), and after upping my accuracy, I only had to fix one set of single sided edges, and then I could solidify:
Reduce the accuracy in your model, then redefine the import feature. This can help a lot when there is a small hole that is difficult to locate visually, by letting Pro/E close the small gaps for you. In many cases you will only have to do a Zip Gaps after this, or just check off Make Solid in the feature properties, you will be all set. Try increasing the accuracy setting in small steps and then Zip/Make Solid.
Here's the rest of the solutions:
You probably already tried this and there are too many edges to see, but just in case..... Are you aware that if you view the thing in wireframe, the closed/zipped edges are purple by default and the open ones are pink? You should be able to identify the offenders. Also be aware that sometimes there are multiple copies of surface patches lying on top of one another. This makes them difficult to identify and fix. ---------- THIS IS VERY EFFECTIVE: (my comments)
Use the search tool, set like this: That will find all the one sided edges, clicking them in the list will highlight them on screen. Of course, if they are tiny, they'll be impossible to find, but this will at least give you an idea of how many open edges you are dealing with. The trick to actually locating them is to create a flat surface, offset form a default datum, that intersects the entire part. Merge that with your large surface, keeping one side or the other, it doesn't matter. Now, try to solidify the resulting quilt. If it solidifies, your problem is in the other half. Move the flat surface into that side until it won't solidify anymore, you've then found your problem area, or at least one of them. You may need to extrude a L section to isolate a small corner of the model, moving each leg to find every hole in the model. Tedious, but effective.