Good advice! Counter clockwise selection of vertices is important as well as being meticulous about getting each surface right. I found this script very useful:
http://www.inivis.com/forum/showthre...iding+surfaces
By hiding surfaces I can look inside a model without deleting anything. Convex surfaces tend to get the normals wrong. A black surface usually means that normals need to be flipped. By the way, I've spent hours trying to fix up a convex surface. Then I found a better way, I cut a slice of a sphere and used this for my curved surface element.