Many manufacturers make adhesives that would work. I use 3M's 8115 body panel adhesive. You would rough up the metal that the flare would be sticking to and the part of the flare that is going onto the wheel wheel with 80 grit. Apply the adhesive and clamp it in place....allow the adhesive to dry for the recommended time and then give it about 6 more hours...remove the clamps and the excess adhesive with 80 grit paper. rough up the outside of the flair and the metal and apply fiberglass filler onto the flare and the metal, making the flare mold into the body. Shape with a block and 80 grit paper. Apply regular filler over top of the fiberglass filler, again, shape with a block and 80 grit, finish with 180 grit and prime.