whats the proper way of composting 2 elements in the same scene in maya and after effects?
i want my background on a pass and my car on another, so:
-i am rendering  the background with primary visibility checked off on my car for the 1rst pass
-and a second pass with primary visibilty checked off fpr my background geometry this time
-premultiply checked off in frame buffer MR options
-i just cant the same result as my beauty pass, there is always a gray line around around my car silhouette

here are the results:
http://www.youfile.net/download.php?file=3f5a37939cbc460aaae87d54f9f9c4fe

and here is the PSD with the 2 images overlayed:
http://www.youfile.net/download.php?file=bc36b33f85bcd943e85d42e6986bf0c5

*i also tried with premultiply checked on in Maya, and for both cases i tried the 2 alpha import options in after effects, same problem.

what am i doing wrong , what is the proper workflow to obtain the exact result from the beauty pass?

Your workflow seems to be the right one and this doesnt looks like a premultiply issue, if so the grey lines would be covering your car in a more consistent way. Just try the following: In the car render pass do not set the primary visibility to off, instead set the matte opacity for your backgrund shader to zero and again render using premultiply disabled.

the thing is that i am using mia_materials so i dont have the mat opacity option like maya materials.
and if i gave all the background a black lambert (with mat opacity set to 0) then the reflection will change on my car.

You can map the result of the mia_material into a maya surface shader outColor and use the matteOpacity. This maybe slows down your render a little bit.