When i get the sheetmetal work done on my sons 69 cuda, I plan on using a thick undercoating in the wheel houses. Lizard skin is the plan there, however I purchased a spray in "truck bed liner" material made by rustoleum in a rattle can to experiment with for the rest of the underside to see if it would be worthwhile to use. I sprayed a scrap piece of metal that I primered then scuffed up. This covered nicely with a fair bit of texture, and is nice and thin which helps with not adding excessive weight. I will likely use it for the rest of the underside except for the wheel well areas. It comes in extra tall spray cans about $8 per can. After it dried I tried to chip it off with my thumbnail. On the area of the test panel that I hand sanded with 180G sandpaper it stayed put, on the area I did not rough up I could chip it off.
Hope this helps
Matt