you need to remove all the paint or rust from around the effected area, by either grinding or using a wire knot wheel on a grinder, when the metal is clean, gently knock in the metal around the rust hole, apply some solder paste heat it up with a welding torch or similar device, wipe it with a moleskin or a piece of clean mutton cloth, which will tin the metal, then apply some body solder (plumbers metal 60/40) and spread it and shape it with a piece wood dipped in tallow, you can then grind it or shape it with a body file (dreadnought file) to the required shape, and then get it painted.
if you use filler gap (with fiberglass strands in it) or ordinary body filler, as soon as damp wet weather arrives it will grow 50 mm, have large blisters in it, and ultimately fall out, and you're back to square one, obviously using pug is quicker and easier, however bodging never produces long term good results, as its not the product of an honest days work.
in reality you only get what you pay for in this world.
Either do as Mick said, or cut rot back to good sheetmetal and weld in another piece cut to fit.
get a large piece of sheet metal and either have it welded on or use pop rivets ! after you have it welded or pop riveted then use body putty around the edges, sand it down smooth, put primer on the metal and paint it !
Down by the bottom of my car has become rusted out on the outside in a small area. I have put a rust protecting spray on it, but the hole annoys me :p What can I do to cover the hole from the rust?
