Etch primer is a paint that is designed to physically bond itself to the substrate to which it is applied. This is achieved by combining an acid with the paint so that the acid microscopically etched the surfaces of the substrate, thus forming a physical and chemical bond between the two. In the most cases a colour will be added to the Etch Primer so that it can be seen.
Etch Primers are for the most part deigned to work with either ferrous or non ferrous metals or in some cases both. It also advisable to use an etch primer on fibreglass and resign mouldings