Funny you should bring this up today, I'll soon be dealing with some chips/surface rust on the Crown Vic's frame. And some day I'll do the entirely-surface-rusted frame of the Tahoe (yeah...some day :huh: ). My other vehicles are OK in this regard, "OK" as in everything spotless and at least spray-waxed if not properly LSPed.
There's a thread around here somewhere in which we discussed this Rust/Undercarriage stuff in great detail, and Ron Ketcham made some very good points. Well worth the dreaded
search.
On most vehicles I just keep things very (and I mean *VERY* ) clean. I've generally been able to clean off the salt/etc. without trouble, but some locales use stuff that's a *REAL* PIA to get off. I mean every square centimeter, every wash. I simply do this to all my vehicles usually even in the summer. Yeah, jacks (I keep three in the wash bay area) and stands... every. single. wash. Note that this has worked fine on the already-rusty Tahoe; it's structurally OK and there's no sign that any of its rust is anywhere near perforating.
If you see a chip, or any rust starting, and they concern you
then deal with them before they get worse.
Spraying rust inhibitors can be tricky as it's *SO* easy to get "pockets" that will retain corrosive stuff and lead to the rust you're trying to avoid. IME many/most aerosol undercoatings are prone to this and it's far better to use an actual undercoating gun at high airline pressure. With good stuff like the ValuGard product. The big trick is to do the inaccessible areas properly, and that's just not that easy to DIY (I got a lot of coaching at an young age from a guy who did it professionally, it's just not as simple/easy as one might think to do it right).
For very tight spots where you need "creep" to get in there (e.g., riveted areas), there are a few products that work well IME, far better than typical OTC stuff. Research ACF-50 and Wurth Rost-Off Plus and Amsoil Heavy Duty Metal Protection (I use all three with great results).
For less tricky applications I've done well with Eastwood's Black Heavy Duty Anti-Rust. Nice stuff that dries black and doesn't stay tacky. For areas you can access, and for those areas that you will see, it's great stuff and really easy to touch up. AFAIK it's the only "rustproofing" stuff in the "like Ziebart" category that dries black.
Paint-Over-Rust products: I started with POR15 and it's...eh.... mediocre IMO. There are other similar products on the market these days that have worked much better for me, including Rust Bullet, Rust Seal by KBS Coatings, and Eastwood's Rust Encapsulator (which doesn't require a topcoat for UV protection like most such products). Note that these products are not all that easy to touch up (it can be done but requries sanding/etc.).
Ya know...I honestly think that most people who area concerned with actual rust-out should just get the vehicle rustproofed by a pro who uses ValuGard stuff. It'll be unsightly, not a "concours-look" undercarriage, but it won't rust away. That corrosive-laden water just creeps into everywhere and for most people, washing it away before it does damage simply isn't gonna happen any more than a proper DIY rustproofing is gonna happen. Where a body mounts to the frame is a perfect example- IMO most people will spray stuff, confident that they did it right, and it'll trap saltwater and years later it'll be worse than had they just washed it every week.
