Abstract: A primer is provided as a bonding base in an extruded hollow aluminum shape ensheathed by a polyurethane foam. Due to the different coefficients of expansion between the plastic and the aluminum, changes in length take place, which cannot be adequately prevented using the known primers.The invention therefore solves the problem to widely prevent the shrinkage of the plastic sheath and simultaneously to provide a way of depositing the primer in a simple manner.This problem is solved by the invention in that the primer is a compound of resin, pigment and acid. The purpose of the acid is to cause the bonding reaction with the metal substrate, whereas the pigments on one hand cause the bonding reaction with the plastic sheath and on the other hand are used to ascertain the apparent film thickness.An improvement in adhesion of 1,000% is surprisingly achieved when using such a primer.