I have a mint 1999 Chevy Silverado. It has 450000 kms on it and runs like it brand new. The one problem is that I cant seem to keep the rust from stopping underneath. Iv tried painting undercoating ext. but it still seems to show through after 3 months. Any ideas to keep my truck rust free for many more years to come. In better words how to stop rust from getting any worse once its already started.