If your spraying clear you wont need to flatten your base, get your mettalic base spray that as per usuall, then spray 2/3 clear coats then on your last 1/2 coats of clear thats when you add your flattning agent, you only need a small amount otherwise it will go to grey,(try a test panel first), try your best to miminize overspray as stated before, you wont beable to cut it out, Ive done the same with a champagne met and it came out mint