These are used to cross-link with the hydroxyl groups of other resins in order to form hard films.

They are derived by reaction between formaldehyde and either urea or melamine, giving urea formaldehyde and melamine-formaldehyde respectively.

Urea formaldehyde is used mainly with a non-drying alkyd (such as castor oil) and may be stoved at 120°C. Finishes are suitable only for interior use. Where the substrate is wood, the curing temperature may be reduced to low or ambient by use of an acid catalyst.

Melamine Formaldehyde can be used with alkyds, acrylics or epoxies and is more a reactive than U/F. The films have better durability, suitable for exterior exposure.

They feature largely in car finishes, as amino/alkyd colour coats, or thermo setting acrylic colour coats.