The best stuff that I have used for filling dented aluminum, such as on leading edges prior to covering and/or painting, is PolyFiber's SuperFil. It is formulated to bond well to aluminum, and is very light. I think it would work well for dented fuel tanks on the Ercoupe as well.
You can learn about SuperFil here: http://www.polyfiber.com/epoxy You can buy SuperFil here: http://aircraft-spruce.com/catalog/cmpages/superfil.php Good Luck! Stan
