I imported the stl into SU and could not get FixSolid plugin to fix the problems. Instead of manually fixing it, I re-created the wallplate next to the other one, using the dimensions as best I could. I exported that wallplate as an stl, and seems to look good. If you're new to SU, I can summarize how I made it, but I'm not a master but have some fu.
Post's attachmentswallplate.skp 76.88 kb, file has never been downloaded.
wallplate.stl 35.04 kb, 4 downloads since 2013-05-15
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Cheers,
Fred
Fredrick C. Hagemeister
http://blog.richmond.edu/ti3d