Ive just removed my old underseal/stonechip off the front arches to do in Gravitex and 2k to match the rest of the underside I've re-done and the Tetrosyl Aerosol stone chip I originally done it in is the hardest stuff to remove in the world. Even with a wire brush on a grinder its hard work. I wasn't expecting it to be so tough. It's not brittle and doesn't chip, either! So thats good if anyone's on a budget.

Never use Hammerite on the underside, though. Its shit. Was so easy to remove. It's brittle so chipped really easily and tore off in big chunks when I removed it..

I've used Gravitex and overpainted with 2k and I'm quite happy with it. I rollered both on with a small edging roller as the schutz gun they supplied with shit and it turned out really well. I like undersides to look textured, though, not smooth. Personal preference I guess.

The 3M stuff is apparently very good, but about 4 times the cost.