Body kit help

Ok guys, I need your help:banghead: I am trying for the first time to make my own body kit for the 2006 Mustang. I have already ruined one body and don't want to do do that to a second one.:uhoh:
The problem I am having is making both sides of the car look the same. I.E. (driving light holes on the front; the wing; side skirts etc...). Some of the modifications I can do with styrene, but some require more complex shaping. I bought some Tamiya (smooth type) epoxy putty to try that. I try to measure where I can, but curved surfaces cause more problems.
So what are some of the things you guys do? This will be a contest model, so I want it right.
Thanks for the help!

add and remove, rinse, repeat. You just have to work slowly and keep looking back and forth. I believe that Home Depot and the like sell contour guages. They are sort of like nails all lined up. you press them in place and they show you the contour

http://www.starshipmodeler.com/tech/jw_vfws7.JPG

Like that. That will allow you 100% or near accuracy. But I think on such a small scale, most all of us just eye-ball it

go slow, and either:

perfect one side, and then modify the other side to match

- or -

go slowly, and do the same modifications to both sides at the same time

Even if the shapes are complex, I'd suggest using styrene. If you laminate several pieces of strip styrene of different contours, you can make those complex shapes. And by cutting two of everything, you can easily control symmetry.

There is no easy way to do this sort of work. You might learn a trick or two, but really it's all patience, discipline, and experince. If you're a perfectionist, this is not the sort of thing where jumping in with both feet works well.

If symetrical imbalances will drive you crazy (and for what artist or craftsman wouldn't they?), start small and build your skill over several builds. Add a smallish lip to the bottom of an existing spoiler (cut an accurately drafted curve out of styrene, glue it on, and add a fillet curve with putty), or try to drill out two identical light holes in an existing spoiler. But until you have a good feel for the materials and techniques, you're in for a rough road with extensive mods.

I'm not trying to be discouraging, but there is a significant learning curve to be able to put out professional looking results on a major body kit. On your first several attempts, don't stack the deck against yourself. (And the added pressure of a contest will in no way help....:disappoin)

Remember the 3D polygon characters from the PS1 era? Even a complex surface can be represented by a collection of flat polygons, and the edges are digitally smoothed.
Idea is similar. Make a collection of flat surfaces with styrene, and file off the edges, and then add a very thin layer of putty as a final touch.

Thanks for the info guys. I will try some of the suggestions here and see what happens.
Thanks!