If I want lite, I just use microballoons; for strength I use milled fibreglass. Sometimes I use both. It depends. But I ALWAYS use my trusty 3M respirator mask. Either product can do nasty stuff to your lungs. One of the fellows down the road from me has a tiny pump imbedded in his chest to pump out the fluid that builds up in his lungs. He never smoked, but he breathed in powder and fibreglass, on the job, 'cause he didn't use his mask. Bummer[]

R.C.