Many surface treatments that improve adhesion have the effect of roughening the surface involved. Treatments such as abrasion and etching remove the weak boundary layer and improve the wetting behavior. Liquid adhesives that are spread during the application do not ensure removal of air from the cavities present on the surface of the adherend. The penetration of adhesive into the pores of the substrate is influenced by capillary forces and is retarded by the viscosity. Surface roughness and surface irregularities can effect the spread of adhesive and thereby its contact angle.