Diecast models have a high gloss finish when new, which is absolutely no use when you need to apply pigments (weathering powders). The solution is to apply a layer of something to present a matt finish for the pigment to 'stick' to. Why is the word 'stick' in quotation marks? Because the fine particles of powder fall into the valleys in the surface of the matt finish rather than stick to it. A gloss finish is smooth and reflects light, which is why it appears shiny, whereas a matt finish is rough and thus does not reflect light so well.
Any matt finish will provide a key for the pigments, but my favoured substance is Testor's Dullcote. The aerosol version is the easier to obtain in this country, so that is what I use. An aerosol is not the most controllable way of applying a substance to a surface in the quantities we use as modellers. The flow is either on or off - nothing in between - and directional control is very basic, too. The method I use to ensure a light coat is applied to something like our Land Rover is to start the spraying off the model and then move the can along the length of it, maintaining the same distance from it, and cease spraying off the other end. This ensures a constant application along the whole length of the model. The volume that falls onto the model will be dictated by the speed at which you move the can along the length of it. The intention is to spray enough to cover the model but not so much that it creates runs and a buildup of fluid at the bottom edges. Much easier to demonstrate than describe!
You are quite likely to end up with air bubbles in the surface, but I have found that the majority of them will disappear in the drying process. Any that persist into the almost dry state (when the shine of the Dullcote starts to disappear) can be dealt with by the judicial use of a pin.
In the case of subjects with glazed areas I don't bother with masking of any sort, preferring to clean up once the process is completed.