mustard eh?
i've been using Windor and Newtons Art Masking Fluid, as recommended by Steve the Swede. it is FANTASTIC stuff!!
you can pick it up at Micheals. it might seem spendy at first, $10 per bottle, but the bottle goes a LONG way. i'm well over halfway through my MSH build(see my thread in the helmet forum) and i've used roughly 1/10 of the bottle.
it dries very fast, about 15 minutes or less, and produces perfect irregular scratches. it is excellent excellent stuff.
if you do choose to use it, i'd recommend picking up some cheapo brushes. because the masking fluid is liquid latex it will dry on the bristles of your good brushes and ruin them. Micheal's also sells a pack of 12 cheap brushes for 99 cents. that is what i used for almost all of my helmet. i started out on the testor's model brushes which work well too. they are just more expensive.