the factory under-sealer was pretty capable untill we had to start using salt hear. the effect it has had on cars is frightening. i don't think you'll ever be able to stop it, but one remedy which seems to be the best is underbody waxing. i've done a few now and it looks very similar to the wax that came on the uk spec series 1 legacy's. its a messy job, but worth while imo. i recently had back in a car that i done last year, its an every day driver and he lives up the side of a mountain. the car was covered in muck and grit underneat, when i cleaned back an area the sealer was still perfect.
i used a mixture of wax they use on boats and rust inhibitor. it was recommended by an old guy i know that has been doing vintage stuff for years and has tried everything. its not nasty like and shultz, it dries an off clear/yellowy colour and could be removed with a diesel jet wash if you turned the heat up where you normally wouln't have it. one pic of the sealer the other is a whiteline bar that the salt got, you can just make out the flat out sticker