How do you get smooth edges?

I'm working on fiberglass doors for my car and have laid quite a bit of fiberglass and built it up with bondo, prolly not the best idea, but the guy that tried doin it the first time didnt do a good job and i'm having to reshape most of it. Its full fiberglass door top to bottom and I'm just wondering how do I get my edges straight and durable? Sorry dont have pics at the moment should have some over the next few days, went to get a camera cord to hook it to my computer and it was the wrong size, takin it back and goin to a diff store. But in general what are the best ways to build up edges to not only get them straight but so they are durable.

Use a block of wood and sand paper to get them straight, as for durbbility I am not sure what you mean, like you said pictures would help.

k I'm bout to go get the wire right now should be up later tonight.