Use enough to cover the entire area with a thin film.
These were at times covered with a thin film of ice.
Think about a very thin film of oil floating on water as an example.
The board is thrown across a thin film of water.
A thin film of recent mud was left upon his skin.
There was a thin film between Hansen and everything around him.
A thin film, usually of a plastic material, is used instead.
This was now covered with a thin film of ice.
Leave a thin film of the medication on the skin.
And the water crept over the floor, a thin film of it.