Duct tape is way to thick. If it closes on a piece of duct tape you are seriously off face.

I prefer to degrease with brakeclean or similiar and use a piece of Scotch tape. Maybe even two pieces layered together.

Anything more and I consider it to much of a 'shim' to shoot it.

Scotch tape is also a good way to see how far off face it actually is. The tape is .002" thick. If it closes on one piece but not two, then you are .003" off face, etc.....

Mike