Matt---I think what you have is a polyester resin bodyfiller with very short strands of fiberglass mixed into it. It is intended, as the name implies, to repair damaged fiberglass, as in fiberglass boats, snowmobiles, etcetera. It will work as a bonding agent, but it really doesn't give that strong a bond to steel.---if you want to use it for a bonding agent, take a 1/4" drill and drill a series of holes about 1 1/2" apart through the good metal, just beyond the rotted area. Take a tapered chisel of some type, put the end against each hole, and give it a light whack with a hammer, thus creating a small "funnel" shape at each hole. When you trowel on the "fiberglass putty" it will squeeze through the holes, and harden, forming a "key" to hold the patch and the rest of the filler on the outside in place.----This is a good "poor mans fix", used by penniless teenagers and unscrupulous used car dealers worldwide. I have fixed many of my own cars that way when I was young, and even fixed up the odd junker and sold them to poor unsuspecting fools in one of my previous lives