Originally, the fender would have been attached loosely to the body with enough room to slip the welting in between the fender and body. Slits would have been cut into the material to allow for the fender bolts. The fender is then tightened. The lacquer covering of the original welting was tacky enough to hole the welting in place while the fender was tightened.
Today, if you were to use the same method, you would have problem. The vinyl and polyurethane materials are slick, making them very hard to keep in place during installation. To start our project, we measured and cut a piece of welting. We added about 6" onto our measurement for positioning of the welting later on. With our piece cut to length, we then notched the welting to match our fender radius.This was done by cutting triangle piece from the welting to the bead.
This photo shows the radius notching and the cool tool used to do the job. If for some reason you haven't seen this tool yet, Sears is the place to get one. Once, the notches had been cut, we test fit our work on the fender radius to see if more notching was needed. The spacing used here will work for just about any radius. Straight sections only need trimming at the bolt holes. They can be trimmed out now or after the welting has been attached to the fender. With our piece trimmed and test fit, we moved on.
Shown in this photo are the 3M #90 High Strength Adhesive that we used to attach the welting to the fender and some thinner for clean up. The best type of thinner for this job is a medium temperature lacquer thinner. This allows for clean up of the adhesive, but allows the thinner to evaporate before any damage is done to the finish. Wipe off any excess adhesive fast and don't rub for too long. Quick, wet wipes pickup the adhesive and allows the thinner to fast off quickly. Apply the spray adhesive to both the fender flange and the welting.
Keep a cloth with thinner ready in case you miss. The 3M spray head is adjustable to spray pattern tight enough not to make a mess. Test your spray pattern before you start. This will eliminate surprises. One or two passes is enough for both pieces. Be careful not to pile up the adhesive as you spray. A thin coat is what you are trying to achieve. Allow a 1-2 minutes for the adhesives to tack up and then apply the welting to the fender. Keep the welting bead flush with the top of the fender as you apply the welting. Allow the adhesive to dry for 10 - 20 minutes before attach the fender to the body.