I think that these Zimmerit sheets are easier to stick to a model surfaces than making Zimmerit yourself. But, for sure the second method is cheaper and possible to be faster with a way I know.The knife is unnecessary.
You didn't want to put a basic undercoat on first? I'm not an armour guy, but would have thought that getting the airbrush into all the nooks and crannies would have proven difficult.
Some Armour modellers build the entire kit, then paint and weather, so I guess there are some small hard to get at areas that dont get any paint!
Usually those places aren't seen anyway...once it is on display.
While others biuld and paint components as they go along..
Each to his own, I guess!