it is very essential before the coats of paint whether it is wall paint, wood paint or iron paint because it works as adhesive between base material & top coat material. Primer is also in two qualities – water based & oil based. We use water based primer In distemper & plastic paints & use oil based primer in which we mix turpentine oil for gloss finish paints & wooden paint (wood primer is also available in the market). Few people think that primer is a optional material & it increases the cost of the painting work but it is a myth because by applying primer on the surface, it reduces the absorption level of the paint & you safe more than 30% of your paint. Generally Paint is twice costlier than the primer.