Advice on Removing Undercoating

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Jun 28, 2020
Bend, OR
Howdy.. I'm looking for help here.. I need to address some patches of surface rust on the bottom side of my truck. I've got the body off and on blocks and am ready to go to town.

But, the factory undercoating is showing itself to be SUPER resilient. I don't expect this to be an easy job, but whatever I can do to make it better would be great. So far I've really only tested a wire wheel on a grinder.. That will get it off but it's super messy and only really good for flat sections. It isn't gonna work for any crevices, etc..

Has anyone got experience here? I've heard of people heating it up and scraping it off (I'll try that this weekend). Perhaps chemical stripper? Or sand blasting (seems like it will just bounce off)...

There isn't any rot except one spot on the front of one of the rear wheel wells, but all the seam sealer is cracking. I'd like to get it down to bare metal, put on an epoxy primer, then a coat of lizard skin or similar.. Then I'll fluid film everything once it's all back together.

Here's what I'm working with:

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One of the best ways is to spray the area with liquid nitrogen and then bang with a hammer. check your area about renting the tank and nozzle. You can also use dry ice to do the same thing. A heat gun and a scraper will also work but its quite a bit more effort and can get pretty use old tools and wear old clothes.
I've had to remove the factory undercoating in that same area, and the best tool I've found is a curved chisel. Something like this:


It has to be SHARP. Cuts through the undercoating like butter. Then I hit the leftover bits with a wire wheel to clean things up. YMMV.
I think trying to remove that tar product is a lot of work for little gain. I would spray the (rusty) crevices twice with Fluid Film and drive down a dusty road. Check it every 6-12 mo. and touch up as needed. If you have places where this tar-like substance has lifted/peeled and has dirt/salt/water behind it, those must be scraped down to solid material (and then coated with grease) to prevent more problems.
I used a torch to soften the undercoating and then simply scraped it off while warm. Messy and time consuming, but satisfying to get down To metal fairly easily.
heres a vid removing the stuff thats on the inside of most cars:

Another option is to plug your angle grinder into a voltage regulator and slow it to a more manageable RPM. I had a similar issue removing spray on bed-liner and I found that using it at about 60% power, the gyroscopic effect was way less pronounced and I actually could get into the nooks and crannies much better. The regulator has been useful with some other tools/projects in case you needed more reasons to buy a new tool. Good Luck!

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