Hi Rockman,
http://www.aallpaints.com.au/shop/product_info.php?products_id=726&osCsid=bfc8a2c1333e87622fd869236b974cac
Back when I was spray painting we used gravitex alot.Sprayed many a ute tub with it and is used to replicate the ripply stone guard finish on the sills of cars .There are other brands around ,any decent panel/paint supply shop should be able to sort you out.You can buy a underbody "schutz" gun cheap enough to put it on.just need a compressor.....
You can paint over it once dry,just make sure its dry and rust free first as you will lock in anything nasty... once its on..its on.Practice on an old bit of ply or something first.DONT SPRAY IT NEAR YOUR CAR.......
Application...thicker = ripplier finish
thinner = flatter finish
Ask away if you need to know anything else,I will help if i can.
Cheers
Pete