im a lil' late here, but i use that technique 
this is an old pic of my helm, in fact the first w/visor on 
any way some scratchs are made with sandpaper and others i cober the helm with tape, but i apply sandpaper to make more smooth 
<img src=http://200.72.163.179/cdref/tdh/hv5.jpg>
the new scratchs are made with a knife :evil and some others just usin' the helm like a mop ...
just kiddin ... but not with the knife part, i try to post new pics 