You might have to use a pliable mask like Blu-Tack. You'll need to experiment to see if it will pull off the Zimmerit, but it should help. Another possibility is to use tape masks, rub them down as tight as possible, and use clear to seal the edges. Also, spray with low pressure and a detail-type paint thickness to minimize overspray. Some of the actual vehicles I've seen photos of look like the overpainting was done with mops, rags, and hand brushes, so the soldiers in the field were not always as neat as most modelers.