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When the filler is hard (usually 15 -20 mins) sand into shape using 40-60grit production paper. Power sanders can speed up this part of the job but care must be taken not to remove too much of the filler, particularly if using rotary sanding disks. Final shaping is best done with a flat sanding block with abrasive paper attached. Sand in straight lines, changing direction regularly to avoid sanding low spots or grooves into the filler. For the same reason never sand in a circular motion as this removes filler too quickly from the same spot. Once the filler seems to be very near the required shape change to a finer grade paper, eventually finishing with 180 - 280 grit.
Unfortunately shaping bodyfiller is one part of the job that takes patience and a certain amount of practice to get perfect.
Small pinholes and low spots are usually filled with `stopper', which is basically just a very fine filler. Polyester Stoppers are used in exactly the same way as bodyfiller, but are meant for use on smaller areas. Cellulose Stopper, or cellulose putty as it is sometime called, requires lengthy drying times between coats and is still prone to later sinkage. Very thin coats are an absolute must, with at least 30 mins between, preferably longer.
Once the filler/stopper repair is satisfactory, final finishing before primer coats should be done using 240 grit WetorDry to remove the last of the deep sanding scratches, flatting finally with 320 grit, A useful trick is to spray a light coat of black or any dark colour (aerosols will do) onto the repair before moving on to the finer grades. This way as wet sanding is carried out, deep scratches or any remaining low spots will stand out clearly as the `guide coat' of dark paint is sanded off. It is much easier to put small defects right at this stage than to find them in the final coat of paint.
Use a rubbing block at all stages, otherwise the surface will have faint ripples which will not show until painted.
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