With the metal as straight as possible, polyester filler (more commonly known as Bondo) is the next step. If you like Chip Foose, Charlie Hutton, Bobby Alloway, George Barris, Tim Strange, or Troy Trepanier's quality of work, then "Bondo" is going to be on your car. It's not a crime or a sin, it just needs to be used properly. The goal is to have it thin-no more than 1/4-inch, if possible. Filler needs aggressive scratches to hold on to-36- or 40-grit is standard. It's likely that 80 percent of your filler will end up in a pile of dust on the floor, but don't skimp on quality. The better quality fillers work down easier and have a minimal amount of shrinkage-nobody wants shrinkage.