>Friends;
>
>    Ramona has rot-holes on both of her rear quarters, on the corners.

How big are the holes? larger than say 2-3" and you should consider having 
metal brazed/welded in.
         You need to sand/wirebrush/hammer off loose rust and the nearby 
paint to get fresh/shiney metal exposed. Do this well, then degrease with 
paint thinner. Paint the surface with rust paint, POR 15 sounds great BTW. 
Apply body filler, long strand type with chopped up fibreglass bits in it. 
It is best to indent/dimple in the surface around the rust hole so that the 
bondo sits well and you can sand it smooth/flush into the metal surface. A 
plastic 'paddle' or spatula makes spreading bondo out easier. Get the 
backside area clean too, this helps bond the patch on both sides. You can do 
larger holes with metal mesh backing, held in place with sheet metal 
screws/pop rivets. 
        Once the long strand filler is hard, sand and shape the area until 
is looks/feels correct. Your fingertips will tell you a lot about how well 
you are doing. You can remove screws now if they are too high. Apply regular 
bondo to fill in pocks/scratches and resand for 'final' shape. Apply primer, 
sand down with 400 grit paper then reprimer and final coat.
         This is cosmetic fix, not structural, so if your battery tray is 
rusting out with the rear corners this won't strengthen it much. Same for 
fibreglass patch repairs. Water is the enemy in either case, if it gets 
behind the metal mesh or FG or the bondo it will start rusting again and 
fall off. Paint seals the outer surface, so primer/paint then undercoat the 
inner side too. There is a Haynes Autobody repair manual, worth looking at 
for info. 

tim s.