I've normally used just an enamel paint ( normally Humbrol ) in a light grey matt finish. Once brushed on, it can be lightly sanded to give a smooth surface and a good "key" for the finishing coat(s).
For filling, mix in a little talcum power and brush onto the panel line /divot / sink mark or whatever, let it harden, and lightly sand back.
Cheap, easy and effective.