As many may already know, I’ve been considering the use of dry ice to remove tar that’s mostly covered by paint on the interior floor.

But my 914 also has a heavy Wurth coating underneath the floorpan.

Will using dry ice inside for tar removal risk removing the Wurth undercoating too?

Probably not if you leave the undercoating undisturbed. It'll warm up quite quickly. You might want to try putting a space heater under the car. Try a small section. Can you reapply more of the Wurth coating if necessary? Are you using dry ice or liquid nitrogen? The later evaporates so fast that the Wurth coating might not give a shit. My 2 cents.

Only considering dry ice at the moment.

My fear is at the same time the tar’s adhesion inside is broken, the same might happen to the Wurth underneath the floor.

This is way beyond my range of experience; it’d would be a nightmare to create a new mess while trying to fix another

Woah. That is a new concept (for me). I know the dry ice is worth doing for the interior tar. Makes things easy. The only even half way worthwhile suggestion I have is to try a test section. That won’t tell you about creeping weakening of the Wurth stuff, but will tell you about outright failure. Might try it where the tar is in the worst shape and replacement of the body Schultz underneath is most likely. Maybe though it is the “rubberized tar”material so often found underneath. Best I can say is I wish I had chosen something else for one restoration, even though it is OEM. Again, the test area still makes sense.

Sorry Rufus,

With what I have seen in your post regarding the pans I would expect the corrosion/rust to have seeped through the seams and would expect more on the bottom.

If I was trying to properly fix those floors, I'd want both sides exposed.

You have to clear as much of the rust as possible, from both sides, and then properly treat, prime, paint, and then re-apply undercoating/floor tar.

If it was me, I'd be HOPEFUL that the dry ice would also remove the undercoating.

I don't think it will, but I haven't tried it yet.

Undercoating is not hard to apply, it's messy but not difficult.

It's a PIA to remove however!

A pre-check tip.

Usually when rust is behind undercoating the undercoating is "loose".

I'd look for undercoating that you can easily "pick up" with a small pick. (harbor freight cheapy set). sometimes when you pick up a corner it "zippers" down the seam where the rust is present.

Freshly done undercoating is less prone to do this.

Fresh undercoating usually means someone was trying to hide something OR a perfectionist has been at work.

Normally the "perfectionist" comes with a shit load of pictures showing the entire job...

Per the PO’s advert:

“- Full repaint in original Tangerine (23)
- Only minor surface rust repair needed in a few spots above left rear
taillight, below windshield frame, etc.).
- No structural rust or other body damage.
- New battery tray - the only body part that warranted replacement. No damage to longitudinal, underneath.
- New Wurth underbody sealant.”

This would have been circa 2014. (My bolding added.)

I’ll check in the next day or two whether the undercoating’s pliable or brittle

If paint shop is worth a you can probably ASSUME they took care of the underbody rust given the hefty bill to prior owner. . . But who knows . . . so Ninja’s point is valid. But, I understand why you are trying to avoid scope creep to the underbody.

However, I’m going to remind you that the tar comes right off with an oscillating scraper. It really isn’t that much work.

And again, as I mentioned previously, if you have perforation, you’ll see it from the inside.

So you really have two choices:
1) Risk disturbing the undercoating with dry ice trying to be like the cool kids.
2) Do it the old fashioned way (mechanical scraping) when the oscillating scraper, some solvent to remove residue, and elbow grease.

Even with the dry ice, you’re still going to have to do spot cleanup and residue removal.

Remind me again why dry ice is so much better?

If worried about undercoating then use an oscillating tool with scraper. That plus a bit of manual scraping and solvent should not take more than an hour.

I have to admit I've NEVER removed floor pan tar with dry ice.

It looks like it would work great, I'm planning on trying it but...

I've always removed in mechanically (like SH) and have found simple mineral spirits as the best solvent. Mineral spirits have to be COMPLETELY removed before any paint can be sprayed and I see the dry ice as a possibly cleaner alternative.

Those oscillating tools have many other uses. You can cut through evaporator housings to replace internals like a boss. Leaves a 1/16 gap that looks machine formed!
They work great for house projects as well.

Want to really see if you have any perforations?

SH is right, but,

If you place your drop light under the car and darken your garage you can really see where the light is getting through from beneath.

I was told you can use Nitrogen to clean the bottom. Would have to be used in well vented area though.

I was on the fence wrt an oscillating scraper vs dry ice. But after seeing pitting in the depression I got scared that mechanical scraping might be too aggressive where there’s thinning from corrosion. That was before considering the dry ice affect on the Wurth underneath.

Maybe the solution is an oscillating scraper on flat sections where rust is less likely and wire brush bits in a drill in the depressions

Not at all worried about elbow grease, cleanup, etc.

P.S. “Scope creep” Haha. I’m so deep down the rabbit hole, I have no idea where I started ... had something to do with debris in the carbs, I think

Thanks bkrantz. The time you mention sounds a LOT better than I was thinking.

A slightly broader question for the group: would I be better off completely removing the Wurth, and replacing it? I don’t love its appearance, and originality’s not as important as long term corrosion protection. So whatever the final surface, it needs to be black, tough / durable, but can’t be spray on. Am I whistling in the wind?

I think you're overthinking it, I’ve done a few with a heat gun and a small assortment of putty knifes/paint scrapers…. You can feel it coming up, it doesn't take much heat, some areas are adhered better than others.

I’d try to manually hit a corner, just so you can see the effort, no heat, a little bit of heat, and so on, if it seems too much, go to the next option….

FYI, I’ve done 4 floors this way, never has it appeared to affect the factory undercoating. YRMV…

You can roll on bed liner, which I think is a reasonable alternative. What people seem to be skating around is the fact that adhesion problems such as rust may also lie underneath the floor and to do a really good job should be addressed. Easy to say for me. I did it for my car, but you end up with your own bills and very reasonably your own goals and standards.

Thanks. Greatly appreciated. I was wondering but hadn’t asked about using heat.

Got more to think about now cause I just found this on Pelican re brushing Wurth on:
http://forums.pelicanparts.com/porsche-911...pplication.html

Worn: “What people seem to be skating around is the fact that adhesion problems such as rust may also lie underneath the floor and to do a really good job should be addressed.” Agreed!

God, my OCD’s killin me!

I removed all of the tar on my floor pan with the aid of a mapp torch and it didn’t impact the undercoating at all.

I would think the process that makes the interior coating come off (Ie make it hard and inflexible) would also work on the bottom coating and cause it to come off as well. I used a oscillating tool as others have suggested. Was a pretty easy job and the tool is not very expensive.

John

I used a heat gun and a scraper and it came off in big chunks. Does not need much heat to break to bond.

Other side of the same coin.

I used heat and a scraper to remove the tar in my 914.
THEN
I used dry ice to remove the tar in my 2002.

I'll never use heat to do that job again. The dry ice came off faster, in larger chunks, with less cleanup after. As well as making the tar very brittle, the freezing causes it to contract and lift off the base metal. Heat can casue the tar to melt and flow or burn if it gets too hot. Not to mention, no tar fumes, no sweat puddles. And if you are doing this on a warm day, its like working in air conditioning being around the dry ice.

Neither method disturbed the undercoating. IMO, there is a clear winner - IF you can find the dry ice.

Zach

I have not used dry ice blasting yet, but I want to, for various things on a 914.

I can't imagine (but I do not know for certain) why using dry ice blasting on the interior floor pans would affect Wurth undercoat under the car. The steel pans may get cool during the blasting process, but without actually blasting the underside, I can't imagine it would affect the underside coating.

I have removed interior floor pan tar using heat and solvents. It was before oscillating tools existed. I remember not enjoying it very much.

Hi Gint - Maybe I don’t yet understand the dry ice process I’m considering. But IF I’m right, it involves resting either bocks or chunks of dry ice on the interior surface until the tar cools, and more importantly, contracts enough to break the bond with the floor pan underneath. Not blasting.

My concern was rooted in whether the heat transfer away from the panel from the other side would also break the bond with the Wurth. From what other say, apparently not.

Gint is talking about Dry Ice blasting. It uses pelletized dry ice under pressure to strip paint off metal and stone. Think graffiti removal. Since its dry ice there is very little mess to clean up. But it is very different from what you are thinking.

Zach

@Rufus My humble option is you go wit the dry ice and remove the tar and then you some more pictures of my 6. One of the first ones made. The coating of the Wurth material is very thick, that is what you see on the bottom of my car before we did the pan. This is what a car with about 100,000 miles? In 16 years looks looks like. Always in Southern California Garaged after 86 the PO I got it from did say he took it to Mammoth skiing. I had the new pans put in and I sprayed it with tan Wurth on the bottom. The Wurth from the factory is thick and on there very good. The point is if you want this right you need to go to the next step that you can definitely do yourself, and afterwards deal with what It's going on the very bottom of the pan if you need to.

Best Bob B


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Look at my blog on my build to see more on the body

@burton73 : Thanks so much for sharing your experience, Bob. Not 100% decided yet, but now tending towards dry ice tar removal inside. Depending on what’s found, then removing & replacing the Wurth underneath if I decide to take that step. Pretty much what you’re thinking

Yep, you have a great car this would be a nice thing to do to it and document it as well so some day people that may continue on with its preservation can know just what was done and in the right way. We are just caretakers for these old air-cooled Porsche cars. You may find the original Eng case someday and just store it like I just did. 17 years looking and it surfaced. An incredible find for me with the help of one of our members .
Found with help of Glenn Stazak and I bought it.
914- 6 eng. case 6404016 for number 41

Best Bob

Just crossed this bridge last week

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Worked well

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I did not check the underside of the floor pan after I was complete with the interior tar removal

Super cool 930Cabman. Looking forward to watching your progress.