I just finished with my trans am today and after all the work I had to do with the rust behind the body kit I don't want it to happen again, now j bought the car just recently but in northern Michigan u never know how the weather will be. So I was just wondering if anyone has used like a sealer between the body and the kit to keep moisture from getting in

 

No, you can't keep water out if you drive it in the rain.
The gfx pieces have holes to drain, but they get clogged with road stuff.
The main cause of rust behind the gfx is the steel brackets that hold them on. They aren't painted so they rust which eventually transfers to the body.
Best thing you can do to prevent rust behind the gfx:
Make sure paint on body is in good condition
Make sure the gfx have the plastic tabs that hold the bracket and seperately it from the outside of the body
Makes sure mounting holes are well primered and painted.
Paint the retaining brackets
Use rubber washers or silicon sealant between the nuts and inside of body

 

Thanks for the information, I had to make new brackets and painted them with paint also used stainless steel hardware, but thanks for the info