Hand sanding is the final surface preparation for most any kind of surface that is to be painted. This final sanding helps clean and smooth old surfaces but, more importantly, it is used to freshen any surface and provides the necessary “ tooth” for better adhesion of the primer (or top coat)
Sanding is equally effective for composite, wood, fabric or metal surfaces. ( I understand that even one of the major aircraft manufactures considers it beneficial to have all their aluminum surfaces lightly wet sanded with number 400 wet/dry paper before undertaking any other preparation treatment.)
Do most of your heavy hand sanding with the help of non-rigid sanding block (rubber or foam) in order to obtain a true surface free of waviness and bumps.
Be careful when using hand-held paper as you may develop an unevenly finished surface because your fingers will tend to exert localized pressures. Hand – held paper also tends to ride over hard spots ( small bumps of dried glue or paint ) and leave the spots virtually intact. To remove a small localized hard spot or lump, use a smooth file or a hard sanding block to reduce and level the imperfection before continuing your hand sanding. Do not sand into composite structure to remove a large reside area. Make the correction in an approved manner.
If a composite surface is in poor condition, begin with number 180 or 200 wet/dry paper and use one of the larger non – rigid sanding blocks to improve the overall surface. I think it is preferable to sand fiberglass surfaces dry. Next switch to number 320 paper for the final finishing.
Try to do all your sanding with straight strokes, either horizontal or vertical, and resist the temptation to sand with circular movement. This is particularly important during your final sanding because a circular sanding pattern leaves swirls and sometimes sandpaper scratches that may later be visible through the final top coat.