Sorry mate, i will reply to the pm in better detail later, proper busy right now! Use really rough paper to start with, about 80 grit. rough the whole area up, try get it back to metal and the existing filler to a point where its solid. Apply the filler in several coats, sanding between (80grit paper on a block) until its nice and level. Sand with 240 grit paper (again, on a block) to lose all scratches. Prime several times, until its lovely and smooth (use a hi build primer) flat the primer using 600 or 800 grit wet and dry paper (use wet). Build up coats of even paint until its lovely and shiny. If it comes out dull, use 1500 grit paper to very carefully flat back the paint. T-cut lots and then polish.
Use P38 easy sand filler. Sandpaper from b&q and an aerosol from halfords.
Hope this helps
