Sandtex Flexible Primer Undercoat. A high performance flexible primer undercoat providing the ideal base for long lasting eggshell and gloss finishes on exterior wood. Specially formulated for use with the Sandtex Trade Flexigloss and Eggshell X-tra finishing systems on exterior joinery, this solvent borne, high performance flexible Primer Undercoat provides superior resistance to cracking and flaking than traditional individual primer and undercoat systems. The Flexi X-tra systems can also be used on suitably primed PVC-u.