Slick Sand & Feather Fill 2G are both EXCELLENT products. As long as they are applied as directed,they will outlast most of US.
I use Feather Fill often for "problem child" surfaces and has gotten me out of trouble MANY time's.
Rember that it IS an activated product as it use's the same hardner as fiberglass so your potlife will be ticking as soon as it's mixed.
CLEAN your gun as SOON as your finished as it will harden up SHORTLY.