Most paint primers, either oil-based or latex, become hard and brittle after about 12-24 months of first being applied.  Typically, the substrate to which the primer is applied (e.g., wood, cinderblock, metal, stucco, plaster, etc.)  flexes and moves as the temperature changes, whereas the primer, when hardened, does not flex or move in the same way.  The primer on the substrate surface begins to crack and peel again.  Furthermore, if the substrate is not sound, any paint applied as top coat over either the preceding paint or an applied primer will not last.  Therefore, the customer can not expect the paint coating to last for nearly as long as the 20 year warranty that some manufacturers claim when the previous coatings continue to fail under the new coatings. 
Thus, there exists a need for a composition for use as a primer that will resist hardening and stay flexible, thereby allowing the primer to move, expand and flex with the substrate to which it is applied