How To Apply MasterSeal NP 100 Sealant Ensure substrate is structurally sound, fully cured, dry and clean. Use [MasterSeal P 173] primer as necessary. If required, install the appropriate size backer rod for the joint. When using 300ml cartridges, cut tip of cartridge for desired width of the sealant bead. Puncture foil using a sharp object. Insert cartridge into caulking gun. When using 20-ounce propaks, insert propak into a bulk caulking gun. Open propak with a razor knife, then attach cone to the caulking gun. Cut the cone slightly smaller than the width of the joint. Fill joint from the deepest point, to the surface. Dry tool joints to create a smooth, concave shape. Refer to the MasterSeal NP 100 technical data guide for more information.