Resins used in surface coatings are mostly polymers of one sort or another and although they differ in a number of ways from the high polymers used to make plastics and synthetic fibres, they nevertheless show some of those properties peculiar to polymers. One of the things that distinguishes polymers from ‘ordinary’ organic compounds is the fact that instead of all the molecules having the same characteristic molecular weight, the molecules present have a variety of molecular weights (because they contain different numbers of monomer units). Instead of talking about a polymer's ‘molecular weight’ therefore, we have to talk about its average molecular weight and its molecular weight distribution.