Repairing/Protecting small rusty areas of vehicle underbody

[bmwpowere36m3]

So here the question for the forum:

How do you deal with small rusty areas (e.g. underbody and "finish appearance" isn't super critical) that are surrounded by good paint and the rust is mostly superficial and can be wire-wheeled to about 95% off? In the past I would scrape and wire-wheel as best I could, then hit it with POR-15 or Chassis Saver... One issue I've run into is the bond of those (rust encapsulators) with areas that are too "clean" or into existing paintwork. Obviously the best, would be sandblasting the areas with a handheld gun and protecting them via normal finishes.

One thought, though I've never done personally: acid/zinc wash > followed by primer > topcoat. I have read that those acids can inadvertently affect the primer and lead to failure. Another thought, has anyone used an airbrush for small touch-ups? A touch-up HVLP both in terms of physical and fan size wouldn't be very practical either and I would need to scotchbrite a much larger area around the "damage" for the paint to adhere. Besides that, anything would need to be brushed on.

Barring that approach, what's your method?

 

[Donald]

A rust prevention spray like CarWell or FF or NH Coatings or Krown.

All of these products are pretty cheap to apply yearly and "creep" into areas where a rock or chunk of ice scrapped it off.

If the rust is 100% gone you can just prime and paint the areas. Then the rust prevention spray.

 

[wag123]

Just spray VHT rust converter on the rusted areas.

 

[Lubener]

Since its underneath,Clean it up with a wire brush and smear grease over it.

 

[demarpaint]

I'd clean the area, wire brush and/or sand it, clean it again, prime it with a good metal primer, top coat with a color close to it. Then once it's dry [a few days] apply a rustproofing compound over it.

 

[wrcsixeight]

Wire brush wheel.. then a.scrubbie sponge and scotchbrite pads soaked in Ospho.
Ospho works, great on rust, bare steel and painted steel. Only lifts.paint along edges of rusty areas.where rust is under paint.

Excellent adhesion of anything to.surfaces prepped with ospho.

 

[Trav]

So here the question for the forum:

How do you deal with small rusty areas (e.g. underbody and "finish appearance" isn't super critical) that are surrounded by good paint and the rust is mostly superficial and can be wire-wheeled to about 95% off? In the past I would scrape and wire-wheel as best I could, then hit it with POR-15 or Chassis Saver... One issue I've run into is the bond of those (rust encapsulators) with areas that are too "clean" or into existing paintwork. Obviously the best, would be sandblasting the areas with a handheld gun and protecting them via normal finishes.

One thought, though I've never done personally: acid/zinc wash > followed by primer > topcoat. I have read that those acids can inadvertently affect the primer and lead to failure. Another thought, has anyone used an airbrush for small touch-ups? A touch-up HVLP both in terms of physical and fan size wouldn't be very practical either and I would need to scotchbrite a much larger area around the "damage" for the paint to adhere. Besides that, anything would need to be brushed on.

Barring that approach, what's your method?

Clean it well with wire wheel, wipe any area down with cheap vinegar then wipe it down with water and dry it well.
Spray this on it, 2 good coats will do.

Amazon.com: USC 2K Spray Max Epoxy Primer Paint Aerosol : Automotive

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Once it is dry cover it with Napa stone guard and let it cure overnight. top coat it with paint and clear if you want but its not necessary.

https://www.napaonline.com/en/p/MSR4003

 

[JHZR2]

Sorry... I’m not seeing rusty areas. I’m seeing bright metal. No holes... no red rust.

Id get some good self etch or cold galvanizing primer, and get it coated, then some sort of 2k paint so it has hardener in it.

If it was surface rust, there are other converters and rust paints thst I’d consider, but that’s not this.

Now, if you removed a good amount of metal,mand it’s thin, I might consider fiberglass and miracle paint or some of the POR epoxy products, but I can’t tell how strong the metal is from here...

 

[doublebase]

I think you’re doing a good job, what I’ve done in the past is paint it and then cover it in fluid film. Then inspect it every three months, knowing that I’ll probably be repainting and covering it every year. If it’s a high traffic area, instead of fluid film, I’d use Cosmoline or white lithium grease (not white lithium grease spray, just the grease). Honestly, white lithium grease just does a great job sticking and staying.