how would you fill a big hole with bondo if it's no a quater panel? do you have to weld a piece of to it and then bondo over or what?

 

yea weld a plate to the back of the " BIG HOLE"
and then make it flush with bondo!

 

actually the best way would be to trace the hole and cut out a piece of material that is roughly the same thickness as what you are trying to fill. You can use a magnet to hold it into place while you tack weld it and then remove the magnet and finish welding the area. Then take a grind and grind down the high spots and finish up with an orbital sander. If you have done it right, then you won't need bondo except for a little bit of surface filler. You just have to be careful when you are welding to not heat the surface up too much, otherwise, it will distort your panel.