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Tips

Undercoating removal

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J

jackag91

Jedi Warrior
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My Car is covered with a really thick (1/2 inch in some places) layer of undercoating. It is the black asphalt looking stuff.

WHat is the best (i.e easiest) way to get rid of this stuff? I am trying to find all of the rust and this stuff is hiding alot of it.
 
I used a propane torch and flexible scrapper. Find a comfortable place work, put a good station on the radio, and go at it. What remained was removed with scotch pads and paint thinner.

Oh, don't wear shorts while doing this.
 
The torch will set the undercoating on fire if left in one spot to long. Go at it slowly until you get the feel for how much heat to apply to soften the material enough to scrape off. A bucket of water nearby goes a long way to appeasing the fear of fire.
 
The torch shouldn't be any problem with a low, "soft" flame. A heat gun, such as is used to soften and remove paint on houses, might be nearly as effective. Like any heat source, you'd want to keep the torch moving and not overheat any one spot...and keep any other combustibles well away.

Amazing what old undercoat can hide, isn't it? and the potential damage caused by same? Two of my Heralds seem to have had undercoating virtually from new. Both were "Personal Export Delivery" cars; not sure it that's a coincidence or not. Anyway, on the one car that I've "explored" in this area, the undercoat did a mostly good job of protection. There are areas of the chassis still displaying clean Pale Yellow original paint underneath that undercoat. Unfortunately, millimeters away from that shiny original paint are small rust holes where apparently the undercoat failed just a bit, water and dust got in, and the remaining undercoat held the moisture in place long enough to do the damage. /bcforum/images/%%GRAEMLIN_URL%%/frown.gif
 
The worst places that were damaged on my car by undercoating was where it was applied under the fender to body joints. Water seeped in, but could not seep out. Metal gone.
 
ISTR Consumer Reports (or similar) "debunking" dealer-applied undercoating some years ago as being worse than worthless; for exactly the reasons mentioned above.

I agree a torch is reasonably safe when used with care; but I would keep a water sprayer or fire extinguisher handy "just in case". There just might be a pocket of oil under there ...
 
I just finished removing the tar type undercoating on my BMW 2002. I used some of that purple power degreaser, a wire brush, and a toothbrush. I was surprised how easy it came off. I was just trying to clean up the front subframe so I could remove it to replace the bushings. I noticed that the undercoating was disolving where the degreaser hit it. Once I removed the subframe, I sprayed the whole underside of the car and the wheel wells, and went at it. I don't know if it was effective on just that particular brand of undercoating, but it worked very well. I bought a whole gallon of the stuff for 5 bucks at the parts store.
 
If the undercoating is really old, sometimes a simple sharpened putty knife and a plastic or wooden mallet is all that is needed to get up under it and it might come off in large chunks.
 
I used a 1/2 width wood chisle and a heavy hammer
to pry off the majority. Paint thinner and green scour
pads took it to bare metal. The procedure takes a long time.

d
 
I've heard dry ice works well. Just be sure you don't frost bite your hands.

I used a heat gun on the underside of my nissan truck.
 
In my opinion, using an open flame anywhere near an automobile is courting disaster. Use a heat gun, set it only as high as needed to soften the undercoating and make sure everything has cooled down before you quit for the day. If you are doing this work in your home, check your homeowner's insurance policy to make sure any accidental damage resulting from your activities is covered.
 
I have used the heat guns with success. I was afraid to use an open flame. If you use a "hot" method, I encourage you to wear leather gloves, a long sleeve shirt (and long pants as mentioned above). Chances are you'll find yourself working underneath the hot stuff eventually so a full-face type plastic shield is a good idea. Once you scrape off the majority of the coating, liberal applications of kerosene, ScotchBrite pads, and elbow grease will get the bottom very clean.
 
I Picked up a heat gun at harbor freight. So we will see how much I get off before it burns out.

I don't expect much for $9.99.
 
I am in the middle of a complete frame off restoration on my '76 TR6 currently. I tried everything I could think of to remove the 30-year-old undercoating including every method mentioned here. Nothing worked well. Too messy or time consuming. Not to mention the toxic fumes. I finally decided to get serious. I took my body tub to a professional sand blaster. He had it completely cleaned free of paint, putty and undercoating in approx. 3 1/2 hours. Cost me $150 for his professional expertise. He did mention that he used sand instead of other "fancy" abrasives. I didn't really care. I have a clean body tub ready for primer. Consider ALL of your alternatives when working on your project. Good luck.
 
TR6BILL said:
If the undercoating is really old, sometimes a simple sharpened putty knife and a plastic or wooden mallet is all that is needed to get up under it and it might come off in large chunks.
Click to expand...

With the added "advantage" of yanking the rust out with it!
 
The sand blasting is a good idea. I didn't think that would work. I figured the undercoat would just get all gunked up.
 
The only problem with sand blasting is you have a very excellent chance of thinning the sheet metal. Sand blasting the outer body panels can easily warp the sheet metal as well.
 
TR6BILL said:
The only problem with sand blasting
Click to expand...
The problem with using sand instead of those "fancy" abrasives is that the dust created can give you (and anyone else that breathes it) silicosis, . Suffocation because your lungs don't work any more is not my idea of a good way to die.
https://www.who.int/mediacentre/factsheets/fs238/en/

Abrasives like Black Beauty are inexpensive, work better than sand, and are cheap insurance, IMO.
 
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