I haven’t used Tamiya dull coat, so I can’t comment directly. I have seen a little white powder accumulation on SS car Z-channel when I got overzealous with Model Master Clear Flat and put down a wet coat. The talc or whatever they use tends to settle next to vertical details, on ladder stiles and rungs, or on grab irons, etc. The best way to avoid that is to use a couple of light passes, let it dry, and it it’s not flat enough, repeat the application. Also shake the can frequently to keep the particulate suspended and evenly mixed.  I never had that problem with Dullcote, and I don’t use acrylic flats because they cloud the paint color. I’ve never had enough powder accumulation to have to redo a car. The slight white residue I’ve seen can be covered during weathering, since it’s already in location that accumulate dirt and grime.

Sometimes Dullcote will cloud if it’s sprayed on a humid day. If it’s not too bad, spray Testors Universal Thinner on the car and let it day, If it’s bad, you can try brushing the thinner on with a wide flat brush and let it dry, but you risk ruining decals if they’re not well sealed.

Nelson Moyer