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Best way to clean factory undercoating

I have a chassis on the rotisserie and am almost done with welding. The body (topside) was sandblasted, but not the under-carriage. The floors are dinged and scraped (which will be straightened)but before I go gung-ho on removing the undercoating, I want to try clean it up. My experience is that 50 year old undercoating is difficult to remove, and as such if it adheres well to the chassis and there is no rust underneath...to leave it. But it looks crappy and needs to be repaired in places where it is compromised (scraped etc) before being repainted. What experience do people have with cleaning and prepping dirty-oily undercoating and with what products? As the chassis is on a rotisserie, it will be difficult to move it very far. My shop floor is concrete as is the sloping apron in front of it...but my driveway is dirt, and the rotisserie wheels are not large.
One thought is to lay 1/2 inch plywood in the driveway, roll the rotisserie onto it and rent a Hotsie pressure washer for a day. Downside to that idea is that the water overspray will get all over the sandblasted topside causing it to flash.

Otherwise I can just use steel-wool and a mild solvent and scrub... which doesn't sound incredibly appealing, but the water /solvent can more easily contained.

Last option would be to take it back to a sandblaster on a dolley, to have them remove the grim with delicate media blasting...but I think having access to the bottom while it is on the rotisserie is more favorable.

Let me know your thoughts.
Old 09-09-2018, 06:36 AM
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With my method, you're challenged because of the bare metal top-side. But, here's what I've done lots of these projects nonetheless. I've had great success with this product (local dealer here): Hot Shot High Power Deg. Conc.

I spray it on with a garden sprayer, let it sit for 2-5 minutes, keep it wet with its product, light scrubbing with a suitable brush, and power wash it off. I'm liberal with it. Keeping it wet is key. Start with it dry. You'll likely repeat 2-4x before your happy, but it only takes about 1/2 per run.

Couple this process with a Hotsie and you'll probably get it done in one pass!

How about masking with the self-clinging automotive masking plastic? It's kinda spendy for what it is but could be an easy way to *relatively* seal off the bare metal.
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Old 09-09-2018, 06:57 AM
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Are you going to be top coating the undercoat once clean?
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Old 09-09-2018, 11:15 AM
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undercoat...

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Originally Posted by tperazzo View Post
Are you going to be top coating the undercoat once clean?
I am not sure. The car 914-6 was autocrossed it's entire life so the floors are a bit beat up probably from running over hundreds of cones. So all over the underside are scratches, scuffs and missing undercoat. The wheel wells are missing undercoating too, probably from running low profile wide tires with lots of negative camber...that scuffed the undercoating off when the inside of the tires contacted the inner wheelhouse. My plan would be to clean the entire bottom of the car (including wheel wells) and then see what shape the undercoating is in...re-schultz the missing areas. Once that is done I am inclined to repaint the bottom of the car to be factory correct as it is a "matching" six in Adriatic Blue.
Old 09-09-2018, 01:05 PM
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The underside of this 76 Carrera 3.0, even with scrapes and nicks, was far too nice to remove so I put in about 40 hours with 50/50 Zep Industrial Purple, red scotchbrite and brass and plastic bristle brushes to clean it. You need to wear a proper mask when spraying the Purple, it will burn your lungs out otherwise.





























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Old 09-09-2018, 01:21 PM
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Old 09-09-2018, 01:23 PM
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Old 09-09-2018, 01:24 PM
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Thanks Shaun! That looks like the ticket. Off to HD for a gallon and supplies.

I guess once I get the bottom cleaned up, I can assess what needs to be re-shultzed prior to painting. I'll get some new replacement filter elements for the respirator while I'm a it.
Old 09-10-2018, 02:48 PM
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Good luck Lars!

On this tub, I am sanding any scars to fresh metal and then brushing on epoxy primer, then sand, prime again and then apply schutz with an acid brush that will replicate the wavy dimpling pulling it up with the brush.
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Old 09-10-2018, 03:08 PM
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Get a product called "Black Jack". You can get it at Lowes and probable any other home improvement store. It's a citrus based cleaner designed for cleaning up tar/asphalt. Another good product is industrial Zep orange cleaner but it gets pricey. I'm not talking about the typical orange cleaners, this stuff will eat asphalt (my cousin uses it to clean his skid steer loaders after paving jobs)
Old 09-10-2018, 04:04 PM
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WOW!!! seems like ALOT of effort to clean all that up.. only to spray undercoating back on it.
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Old 09-11-2018, 06:22 AM
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Shaun,

Nice work and thanks for sharing the photos.

can you post photos of the brushes you used/ product #'s
Old 09-11-2018, 09:23 AM
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Thanks. I used a lot of these:

https://www.harborfreight.com/3-piece-soft-grip-wide-brush-set-66812.html

and these

https://www.harborfreight.com/6-pc-detail-brush-set-62616.html


Steel is fine for the first go-around but I switched to brass for the majority of the work once it was getting clean. Plastic at times too.
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Old 09-11-2018, 02:27 PM
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Quote:
Originally Posted by Shaun @ Tru6 View Post
Thanks. I used a lot of these:

https://www.harborfreight.com/3-piece-soft-grip-wide-brush-set-66812.html

and these

https://www.harborfreight.com/6-pc-detail-brush-set-62616.html


Steel is fine for the first go-around but I switched to brass for the majority of the work once it was getting clean. Plastic at times too.
Thanks- My winter project !!
Old 09-11-2018, 03:40 PM
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Best way to clean factory undercoating?

a good degreaser and a steamgun... leaves you with only little extra brushing, does'nt take an hour.

of course... widely cover your driveway and wear a one-time-jumpsuite and goggles.
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Old 09-12-2018, 02:10 AM
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Quote:
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Best way to clean factory undercoating?

a good degreaser and a steamgun... leaves you with only little extra brushing, does'nt take an hour.

of course... widely cover your driveway and wear a one-time-jumpsuite and goggles.
My winter project will be round two in an attempt to remove POs handywork of the black spray on stuff.

Do not have steam but I have used a preasure washer, soaked undercoating with gas, kerosene, acetone, DEP citrus cleaner etc. Removed a lot of it but I spent at least 30 hours to get +/- 60% off. I'll admit I am a novice at this. Time to take the car appart and do it correctly.

Shaun, you mention using an acid bruch to apply worth to get correct wrinkle- can you guide me on that brush? Thanks
Old 09-12-2018, 07:44 AM
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Here you go. These indispensable when applying nearly anything to anything else.

https://www.amazon.com/Harbor-Freight-Tools-Horsehair-Bristle/dp/B006ZBD95Q
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Old 09-12-2018, 02:09 PM
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Quote:
Originally Posted by speedo View Post
Thanks Shaun! That looks like the ticket. Off to HD for a gallon and supplies.

I guess once I get the bottom cleaned up, I can assess what needs to be re-shultzed prior to painting. I'll get some new replacement filter elements for the respirator while I'm a it.
I moved away from Zep, simple green, purple power, etc, etc a few years ago. Awesome from the dollar store is the way to go. Better product by far and cheaper. Much, much cheaper. It's a great degreaser/parts cleaner.
Old 09-12-2018, 04:18 PM
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So I picked up a gallon of ZEP purple industrial yesterday from TSCo. I'll take some pics of what I have to work with and report back. The gallon was only $10 and is supposed to be diluted to create 10 gallons of the purple juice. I'll experiment with the strength. Not the most exciting thread I have every started, but important for me as I want to start with a clean slate on the bottom of this car.
Maybe not clean down to bare metal, but clean enough to re-apply primer/undercoat and paint as required.
Old 09-13-2018, 10:27 AM
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I'm at a similar point in my '81. I wanted to take it to bare metal, but I'd be happy cleaning and a fresh coat of primer / paint.
Old 09-17-2018, 02:10 AM
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