1G What treatment to apply after removing rust from underbody panels?

[XC92]

After removing everything that's bolted on from the rear end of my '92 Talon in order to overhaul it all, other than the strut assemblies (which I rebuilt last year), I'm now removing all visible rust from the underside body panels. It's actually not that bad, mostly rust spots along the seams and openings and nothing structurally bad. I'm using a die grinder and rotary tool with various abrasive attachments to remove it.

When I'm done, and after degreasing, rinsing and drying it all, what should I coat the parts I derusted with, to prevent new rust from developing, to the extent possible at least? I don't mean a final layer such as fluid film or even undercoating, that goes on top of whatever you apply to the metal. I mean a permanent first layer on top of mostly clean bare metal, e.g. primer, rust converter, phosphoric acid, etc. I intend to then paint over that, and then MAYBE apply fluid film or undercoating on top of that.

But what should the first layer be? Off the top of my head here's some options:

• Phosphoric acid such as Krud Kutter or Naval Jelly to convert and neutralize what little remaining rust there is and protect the metal
• Zinc phosphate solution such as POR-15 Metal prep to do the same, plus zinc
• Cold galvanizing zinc spray
• Rusty metal primer
• Clean metal primer
• Self-etching primer

Anything else? Yeah, I'm probably overthinking this, but it's not often you remove everything from the rear end of a car and can do this with such easy access, so I might as well do this right and never have to do it again.

Btw I thoroughly derusted all the parts I removed, subframes, arms, supports, diff, etc., and painted over or will soon paint over mostly with POR-15 (high heat version for the calipers), except the diff, which I painted with Harbor Freight "Grill Paint".

Probably should have applied POR-15 on that too, but oh well, it's done and time will tell if that was a mistake. Really messy stuff, and despite taking precautions I still got some on myself, but it's slowly coming off naturally.

 

[DogWhistle]

Lol, yep! Any POR-15 on your skin is there until you shed that skin. Kind of a mechanic’s tattoo.

You did the right thing by derusting and using POR-15 on those parts. It sucks any remaining moisture out of any remaining rust pockets to stop the chemical reaction needed to form more rust.

We live in the north where there are humid summers and salty winters, and although we don’t drive the DSM in the winter, we have other vehicles that we do and they need constant attention. We use POR-15 for most rust situations without using their metal prep, or anyone else's metal prep.

A criticism of POR-15 is that it doesn’t stick as well to bare clean metal as say an acid etch primer. We’ve only experienced this on brand new metal parts. Anything we’ve cleaned and prepped usually has some kind of surface etching from the rust enough for the POR-15 to grip really well, again, with no additional chemical prep, other than a solvent to clean all the oils and dust off.

Secondly, POR-15 dries like epoxy and whatever runs or drips are there when it’s wet will be hard lumps when it dries. It usually takes more than sandpaper to level out any mistakes.

Thirdly, POR-15 breaks down from UV light. Even though it is under the car, you are correct, you need to top coat it. We’ve had great results with Rustoleum, which is basically a high quality enamel.

Although it tempting in our climate, we don’t undercoat our vehicles with rubberized coatings or fluid film. They can form pockets for water/salt to be doing unseen damage before it’s noticed early on.

Congratulations on having the opportunity to do this step that most DSM owners would love to be able to do.

 

[XC92]

Actually, I'm referring to the underside body panels and frame that the structural and suspension parts I removed attach to in this case. I don't need or intend to apply POR-15 to those, as it would be overkill. I'm looking to remove as much rust from them as possible, prep, then paint. Nothing fancy or super hard. If 30 years of driving in mostly humid and wet regions with lots of snow, salt and all that has only resulted in relatively light rust on them, I see no need to overdo it. It's the parts I removed, which are closer to the road and get the brunt of all that crap, plus kick-up gravel and such, that had a lot more rust and needed something harder like POR-15.

And, I'm just wondering, after derusting, degreasing, rinsing and drying these panels and frame sections, what should I use as a first layer to seal, neutralize and protect against future rust, before topcoating with paint? I'm leaning towards Metal Prep, which is phosphoric acid plus zinc, then maybe a cold galvanizing zinc spray like CRC's, and then a top coat of the suitable color. I have all of these on hand, along with other products. Just not sure what would be ideal for northeast conditions.

 

[DogWhistle]

POR-15.

 

[XC92]

Yeah, not gonna happen, for these parts, for the reasons given. Plus everything there is white or off-white, and I don't think their colors would match and top coating POR is a pain and it's super high-build and stands out.

Again, none of it needs something super heavy duty, just paint, and something in-between to protect against light rust. Think of it as being closer to body panel painting than underbody parts painting. I don't think that body painters would ever consider POR-15 or a comparable product for that.

 

[92AWDHX40]

Por 15.

 

[XC92]

Ok now you guys are just messing with me.

And in case I wasn't clear, I'm not stripping and repainting the entire underbody, just some rusty spots here and there, mostly along seams, spot welds and openings, around 5-10% of the entire surfaces, tops. POR-15 just seems like overkill for that. I'd barely use 1/4 of a 4oz can and then the rest goes to waste as it dries up and hardens. I'm going to paint it with spray paint. I just want to know what to put between the bare metal and paint.

 

[DogWhistle]

You seem to want to get a certain answer.

POR-15 comes in 1 oz cans. We get them in a 6 pack. There is a trick to prevent what's left in the can from hardening that we learned online a few years ago: Try to keep as much product out of the lip of the lid while using, but before reattaching the lid, take at least a double layer of plastic food wrap and place it between the lid and the can and press the lid on. Not only does this prevent moisture from getting in the can and hardening the paint, it makes lid removal possible, because now the paint doesn't act like a glue. Use new cellophane each time you use the can. Buy foam brushes and throw them away after use.

If you are trying to reach into body seams, a spray won't protect much better. Go above the seams and drizzle it down into the seam for best coverage. There is a recent build thread on here where the member took a wand and sprayed the inside of his rocker panels.

 

[XC92]

Actually, the smallest can they have is 4oz. I bought a six pack and have 2 unopened ones left, and a tiny bit left of another one that I used the saran wrap method on. So I could try this.

But, again, I really do think it's overkill for very minor rust that's mostly gone on parts of the car that because they face down or sideways, are fairly high off the ground and mostly shielded from splashes and flying gravel by parts installed below them. These are stamped sheet metal panels, not heavy duty forged or cast parts or frame members. They're not structural and bear no major loads. And my understanding is that stamped sheet metal generally gets different kinds of rustproofing, primer and paint treatment.

Let me put it this way. Would you use POR-15 on exterior body panels? Probably not. And if not, what would you use?

 

[DogWhistle]

We actually have used POR-15 on exterior rust spots below the beltline, where rock chips got the salt working, before I could get to them in the spring. It did a great job of stopping the rust that couldn’t wire brush away, filled the rust pitting, and helped level the treated area. Then sanded it and primed it with a self etching primer, since some of the area around the POR-15 was exposed, non-rusty metal, and painted with body paint, then 2k clear. You cannot find the areas repaired this way.

Best of luck on your project.

 

[XC92]

If it was a much lower section, like the pinch welds, which I intend to get to eventually, I'd use something like POR-15, KBS, Chassis Saver, etc., or epoxy primer and topcoat. But it's way further up on fairly good condition metal, most of which has the original paint looking like it just left the factory, so I really do think it's overkill for this particular application. I'll probably go with Metal Prep, zinc spray and clear coat, but I'll think about it.

Frankly, I've been restoring this car for nearly 2 years and this latest phase has been nearly a month, so I'm pretty exhausted and just want to get it over with. Of course, that's when you start making mistakes that cost you. So maybe I should take a day or two to weigh the options.

 

[1990TSIAWDTALON]

Self etching primer.

 

[XC92]

I've actually learned the hard way that even if you remove every last bit of visible rust, chances are that there's still some in there, so small and inconspicuous that you can't really see it, or hidden in some hard to reach crevice.

So after mechanically removing all the visible rust, it's essential to treat the metal with some sort of phosphoric acid solution, for four reasons.

One, to remove as much of that remaining rust as possible. Two, to convert the rest to iron phosphate and thus neutralize it. Three, to etch the good metal in preparation for whatever primer and/or paint you then apply. And four, to prevent flash rusting if you can't prime and/or paint right away.

I didn't do this when I tried to remove and paint over a ton of rust a couple of years ago on my Talon, and it came back. Well, I also didn't remove all the visible rust, as they were control arms still on the car. This time I removed them and everything else and mechanically removed just about all the rust. Then I treated with Metal Prep, which has zinc and phosphoric acid. Then I applied POR-15.

I just did the same with the underbody. In fact I treated with phosphoric acid three times, once with Naval Jelly, which was easier to work with because it's a gel, then rinsed it off, then with Metal Prep, then rinsed it off, but it flash rusted, so I then applied Rustoleum Rust Remover Gel but left it on a while, then wiped but didn't rinse it off. When it didn't flash rust, I then sprayed on CRC Zinc spray.

Hopefully it all takes and the rust is gone for a long time. I just want this project to be over and to have my car back.