Most fiberglass bodies are made of multiple layers of mat, with just enough resin to saturate the mat. You can buy mat in different weights (thickness). The Bondo product takes 3-4 layers to add up to 1/8". I would never sandwich steel or aluminum with fiberglass, to make a part stronger. The rates of expansion are much different between the materials, with the steel being far lower than the fiberglass or aluminum.

Fiberglass body builders found out long ago that glassing steel reinforcement to the inside of a body would result in read-through, where the reinforcement would show through to the outside.

It's always more expensive to buy resin in quarts, but if all you need in 1-2 quarts, a gallon would just go to waste. It doesn't keep forever.