You will need:
1. Plastic body filler and hardener (Make sure the type used is made for automotive use. Automotive filler is not rocket science to sand. )
2. 35mm specifically edged putty knife
3. 100mm specifically edged putty knife
4. Cork scrubbing block. approx 125mm back button 75mm
5. Block of chiseled wood, approx 300mm a good x 75mm wide 25mm thick
6. 40 grit drilube sandpaper
7. Combined board. [Piece of hardboard or similar, about 300mm square.)
Mix up some plastic body filler on the mixing board, according to the manufacturer's instructions. Apply smoothly over the rusted areas using the wide putty knife on large areas and the narrow knife on small, hard to get at areas. Use the wider knife where possible. Try and get the desired shape before the filler goes hard. Remember, all excess must be sanded off. Some last minute trimming and shaping can be done whilst the filler is passing through its 'cheesy' stage.
This lasts for about five minutes after the initial set-up. Use a slicing action with the wide putty knife. Usually several applications must be made. The time taken for the filler to reach the sanding stage will vary according to the air temperature. Around fifteen minutes in summer, and up to an hour in winter. Placing the vehicle in warm sunlight will speed-up the curing.
When the filler is hard and has lost its 'cheesy' feel, sanding can begin. All fillers tend to form a gummy surface which clogs sandpapers, but once this is removed most will sand freely.
Remove this gummy surface using the cork block and 80 grit drilube sandpaper. By folding a full sheet of sandpaper lengthways and then crossways, it can then be torn into four rectangular smaller sheets that will fit the cork block nicely.
Small filled areas can be sanded level with the small block. It is very important however to use the 300mm long block of wood to sand the larger areas. This will avoid ripples, low spots, high spots etc, that spoil so much amateur work.
Inwardly curved areas can be sanded by tightly rolling a large magazine lengthways, taping it to prevent unrolling, then wrapping a full sheet of sandpaper around and using that as a sanding block. Small intricate areas must be done by hand.
It is vital that all the filled areas are exactly level with the surrounding surfaces, and that when the palm of the hand is rubbed quickly across the filler in any direction, it feels flat and smooth. Keep adding filler and sanding, until it feels perfect. Paint will not hide anything; rather it shows up faults much worse.
Pin holes in the filler may appear after sanding. These are caused by sanding deep into a thick application of filler. All mixed filler contains air bubbles, however, these are expelled by the smoothing action of the putty knife on the surface of an application, but tend to remain in depths past 6mm. Pin holes are best filled with plastic filler - primer, or putty, is not satisfactory..