I use Schultz underseal and Waxoyl underseal too.

For the waxoyl I use a Schultz gun as it's quite goopy and thick. For the "rubberised" stonechip type stuff I use a cheap suction fed spray gun set on full - It opens enough to get the paint thru and give a nice even coating. The best thing is that the gun doesn't need cleaning after (providing you are only using it for underseal) as the paint doesn't set inside it - even after a few month