I prefer 3M™ Underseal™ Rubberized Undercoating, #08883, fur fenderwells, or anywhere you want some scuff protection and a bit of sound dampening. It sticks a lot better than Herculiner or any of the other off-the-shelf bed liner materials, you can apply it to bare metal, and you can paint right over the top of it, and best of all it comes in a nice aerosol can. It looks more like factory undercoating, and lasts a nice long time. It cost me about $8 a can last time I bought it, and that's enough to do two fenders.
