Some plastics ooze chemicals that will repel paint and adhesives if not properly treated. The students thoroughly sprayed the repair area with 3M Automix 5907 adhesion-promoting spray, scrubbing the area with a Scotch-Brite pad instead of a shop towel. Then they clamped the bumper to hold it in place and applied mesh tape across the crack. This provides a substrate to hold filler material across the open areas of the crack. They filled in the backside of the crack with a rough application of filler. Once it dried, they removed the clamps to begin work on the front side of the bumper.