The single most thing most thing I hate when it comes to cars even more then electrical gremlins and torn timing belts (on interference engines) is rust.
Being from Ontario, we have cold snowy winters where salt is lathered liberally EVERYWHERE. This will rust your car an exceptional amount in just one winter let alone 20+ if you have an e30. Below you will see what a not-so-bad or even fairly good condition E30 looks like here in ontario… and even then its rusty. You can undercoat all you want but places like under the carpets will always continue to rust from the inside out. What my opinion is to everyone is to take some time and take your car off the road to tear it completely down so you can see all the metal and to coat it all with a rust inhibitor such as Rust Bullet or POR-15. This stuff can even go over rust it “kills it”, HOWEVER I highly recommend doing it the right way which is to cut bad metal out weld new metal in then coat it.
For a comparison between the brands go here: http://www.drjing.com/Mini/BLOG/SideBySide.htm
My preference is Rust Bullet as to me it seems the strongest and takes very little prep. However POR-15 has amazing reviews also.
These pictures are courtesy of “DR.ZED” on www.maxbimmer.com
This is what a relatively good condition (Ontario style) E30 floor looks like with the carpets off:
After Rust Bullet: