The layer should be as thin as possible but it should also be a solid colour. Say it's AS5, the layer should be a solid grey, and as smooth as possible. I find that a credit card makes it too thin, plus it ruins the card.
I usually put a tiny drop on then rub it with a finger, then slowly add more, until the whole chip is covered. Then I clean off the edges, as AS5 is conductive and I don't want it touching the motherboard.