The most important step for preparing a truck for a spray-in liner is cleaning all debris and waxy or oily residue from the bed. Cleanup should happen during the initial preparation and then after scuffing up the bed for the polyurethane to adhere more directly to the metal.
Leaving any loose debris under the polyurethane when it is first applied can create weaknesses in the liner, and it can also cause bubbling or cracking. After the initial cleaning is done, mask off the edges of the bed that are not receiving the treatments to prevent the material from getting onto the rest of the truck. Once the area has been marked off, scuffing up the area with rough sandpaper--where the material is to be applied--gives the polyurethane a better surface to which it can adhere. The final step before spraying material is to apply acetone to the bed's surface and wiping it off after a few seconds to help remove paint and other contaminants.