Surface rust - paint vs. undercoat

[autumnwalker]

Vehicle with mild surface rust on the undercarriage - frame, axles, etc. Will be driven year round, through salt.

Do you go through the process of grind / sand / paint or do you just hit it with some sort of undercoating (Krown, etc.)?

[autumnwalker]

Apparently you can just Krown over it?

 

[Powerram]

Rust check is my preference on undercoating, here is my experience with rust check, it is from hundreds of examples from every type of undercoating you can imagine. Being in the autorepairtrade the ones with the rust check stickers were the ones in the best condition, the others were good, but rust check seemed to have the advantage on cars 20 and 30 years old. Crown would be my second choice,  Other cars that get the grease and graphite  and thick stuff, it prolonged rust, but gets so thick that it hardens and flakes trapping moisture and debris /salt silt behind the undercoating and accelerates the rotting process. Rust check also has an inhibitor that will stop rust from going anyway further. Another advantage of rust check is it doesn't deteriorate the rubber bushings or door seals , iv seen some cars come in with saggy door seals, saggy hood seals and suspension bushings that are pushing from the joints because of the chemical reaction from petroleum based products. Like getting transmission fluid on a rubber floor mat, it changes the properties and gets all gooey and swollen . I recommend treatment every year of winter driving. Also another observation, my mom bought a brand new truck in 2013, Chevy Silverado, she rust checked it the next day and faithful every year in the fall. I noticed that it was "rusty flaky " on the frame while doing maintenance to it , told her  she should inquire to rust check because they warranty,   they said it was what was under their coating that was coming off, went to gm about it, they looked at it and decided to repair it,  since it was rust checked from new , they removed the cab , box and completely stripped the frame , painted the frame and when finished sent it down to rust check and had them re treat the truck. The truck was 5 years old at that point, pretty impressed that they recognized it was rust checked and decided to do the repair because if it. Those are just my experience with rust check. 

 My Nissan has some thick undercoating on it, going to remove as much as I can, paint as much as I can and then rust check it. 

[autumnwalker]

Thanks for the insights @Powerram!

My wife had her car done with Rust Check since new (2008 Civic) - no rust to speak of, but the door seals are all swollen and falling out of it. I tried Krown recently on my truck and (almost immediately) the door seals started to sag - I was very disappointed, seemed to be the way with all of it. Interesting that you have found differently in your experience with Rust Check.

Really interested in the rust inhibitor in the Rust Check - that would be ideal for my use case I think. AFAIK the Jeep has some of that thick / hard crap on it now. Perhaps my best plan is to get that stuff off, paint, then get it undercoated.

[sar4x4]

My brother has Krowned his daily driver since new.  
12 yrs and 400K* km, no rust....

[autumnwalker]

Thanks @sar4x4! What do you do with the Burb?

[sar4x4]

17 hours ago, autumnwalker said:

Thanks @sar4x4! What do you do with the Burb?

I meant 400k+ km... on the Cummins truck.

I Krowned my Nissan Rogue.  200k km, 10 yrs.  Only rust was surface rust on a strut/brace above a plastic gravel shield under the engine.  I noticed it when I changed the radiator, and sprayed it from a can of Krown.

I Krown the ‘Burb about every 5 years.  It doesn’t see any/much salt.  Except for a couple of panels that had rust that had started on them 30 years ago and I had to replace, she hasn’t rusted a bit!  I swear by Krown!