Complete the account of fat media.

An oil film cures by taking up oxygen and polymerising, so it gains mass rather than losing it. The measure of how unsaturated an oil is, and therefore how readily it dries, is its iodine value. The rule that each successive layer must carry more oil than the one beneath it is called fat over lean. A finishing layer that is by definition removable is a varnish.

The last blank is a definition rather than a description. If a final layer cannot be removed without taking paint with it, it is not functioning as a varnish however it was sold, which is the whole argument for the synthetic resins that were developed to stay soluble for decades.