Martin
the "flat" surfaces of the M1A1 are covered with a non-slip coating before the paint is applied. A number of different methods can be employed - dissolved putty in the airbrush at high pressure, Mr Surfacer 500, white glue dabbed on with a stiff brush, or my favorite - yellow carpenters glue with model RR snow. this stuff is small round grannules and when mixed with the glue - thinned properly, can be brushed over the surfaces.
On the M1 almost all flat surfaces are covered, look at photos of the real tank and you will notice, around the drivers hatch the coating is not uniform and the area where the hatch swings is not coated.
Also, dont forget the steel wire spings that hold down the front fenders. A real simple add on -