I pulled the undercover off yesterday to do my first oil change and change the transmission fluid/filter and discovered a significant amount of rust on the front crossmember. It appears that the foam pad attached to the undercover was trapping water against the crossmember. I scraped off a lot of the rust and am going to wire wheel the rest away, then hit it with naval jelly to remove what rust is left and then paint it.

There are a couple old threads on this subject, but it wasn't something I had thought to look for. Just thought I'd let you all know to check for rust on that crossmember the next time you get your oil changed!

 

wow that is shocking! I just had mine off a couple weeks ago and looks fine, but I'm glad to know that is something I'll need to keep an eye out for! Thanks for sharing!

adam

 

I noticed the insulation on my undertray was torn and retains some water too. Next time I do an oil change, Im going to just remove that foam crap.

 

I had the under tray off to do work behind the front bumper recently and noticed surface corrosion all over the front sub frame, although not quite as bad as yours John.
Check the front part of the sub frame as well which is above the smaller front valance/under tray as well as the brackets supporting the suspension height sensors.
I've heard of the height sensors just dropping off due to advanced corrosion of the brackets.
So I did what you suggested, clean, wire brush, rust converter and matt black paint.