I used a Schwartz type underseal gun to apply the Gravitex. Gravitex is made by U-Pol and seems to be hard to find but I got mine on line from Auto Paint and Panel.
The Gravitex is cellulose based and dries very fast. To get an original looking finish I set my gun to about 60psi and tried to get a thick coating on in not to many sweeps of the gun. Overspray gives a sandpaper finish but luckily due to its consistency you don’t get much.
It covers over the original stonechip finish but I tried to cover some old oil based underseal which although it seemed ok in my tests didn’t quite work. I think was due to the thickness I applied, some thinner coatings applied a few times would cover it but wasn’t that great – better to remove all traces of oil based underseal.
I also found that the seam sealer would have been best primed before applying the Gravitex as it lifted off in one or two of the corners.
I bought some more Gravitex so that I can complete the fixes. I used 4ltrs of the stuff to cover the sills and wheel arches. I didn’t cover the center from the cross member backwards (basically the propshaft area). I guess that would take another 1ltr.
So, some areas to fix due to inexperience but generally this is looking like good stuff. As an update I cleaned off the area that had the underseal and resprayed the Gravitex but the new bottles were thinner and gave a smooth finish. Sod’s law isn’t it – when you want orange peal finish you can’t get it!
So just in case you’re trying to do a similar thing then 6ltrs should give a good coating over the underside and inner wing wheel arches (as in the picture, not inside the outer wing). Also, for good adhesion you need to give the metal a good key, I used one of those surface prep (0.5″ hard pot scourer type thing) disks for the angle grinder – however I didn’t do all the surface; bit of a rush to get the stuff on and I forgot 🙁
Oh, and for the record this is the black colour, the pictures are taken with flash so its looks a bit light.