There might be a type of contact adhesive in a spray-can that would work, but all of this that I have used before comes out too thick to do a good job on a decal. Perhaps someone can recommend a product. Otherwise I suggest using contact cement that is meant to be thinned out such as Barge. Its very weather proof and doesn't turn to goo when the sun hits it. Use the thinner that you can buy with it and thin it down as much as you can according to the directions. Then spread a very thin coat on both surfaces. (Perhaps a paint gun would work if you could get it thin enough. I have used this process in gluing plastic laminate to cabinets, but I used a thicker mixture that was more like what you would get out of a spray can.) Let the cement set up and then carefully stick it on. You will have only one chance to get it right, so you have to do it right the first time. Using some wax paper helps a lot in these types of situations, and it will probably be a two person job. I wouldn't worry about taping off the fender too much when applying the glue since any residue can be cleaned up with the thinner (provided it doesn't melt the decal.) I've got a gallon of Barge cement & some spray guns if you are interested. Walt