Jon, you can do the zimm either way. If you put the clamps on first, you can wipe, scrape, chip off (dependant on how hard the putty is) any excess from the tool clamps. You might bend or dislodge some doing it this way, so you'll have to be very careful. But they should be fixable.
Another possibility is to stick the clamps into the putty while it's still wet. I think I tried this on one clamp on my tiger and it worked okay. I have images in my mind of the parts popping off, though.
A third way is to mask where the parts go. I did this for the tool racks on my Panther and it worked well. Problems are that you can easily scrape off the masking tape when you're texturing the putty. You want to remove the tape as soon as you can after the zimm is textured. Also, real small masks pull up or off very easily. I haven't been able to make these work yet. I tried adding "chiped off" areas to the zimm by putting down irregular shaped pieces of masking tape before the putty, but the pulled up when I textured, too. I am going to try that again on my next one, though.
Good luck with it. Let us see how it goes.
Bill
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DML M4A2 Red Army
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-- There is a fine line between "hobby" and "mental illness". (Author unknown)