3M professional undercoating

[AydinZ71 16]

Hi guys,

 

I have been using a lot of this stuff under the SCCA race car restoration. I have to say, im very happy with it! I get down to bare metal, primer, then put this stuff right on. It goes on THICK, only one coat is needed. it does not chip or peel, even after rough handling of the removed suspension pieces during reassembly. For example, i refinished my mustache bar in enamel spray, and during reinstallation I already have several chips. None with the 3M coated parts. I have not run the car with it yet, so no long term data. However, the reviews online are great. its $12 at Autozone, and is much more convenient than getting a spray nozzle under a car on jacks, or taking the car to a shop to be sprayed. I'm on my third can, and I think I can coat the entire underbody in stages for less than $100. You will need a respirator as it is a tar based product, and it does take some time to cure. In my experience, at least a few hours before it is stable enough to touch but this is highly dependent on your ambient temp and humidity. 

 

https://www.3m.com/3M/en_US/company-us/all-3m-products/~/3M-Professional-Grade-Rubberized-Undercoating/?N=5002385+3292638249&rt=rud

[NCchris 10]

Because of numerous good reviews elsewhere and this post, I have landed on this product for the bottom of my Z. I would kinda like the bottom of my car to be the same color as the rest of it. Do you think this stuff is paintable? Currently, the bottom is freshly coated in epoxy primer after sanding to bare shiny metal. Now is the time to do whatever I am going to do. So, paint it or leave it black?

Thanks for any info.

Chris

[rossman 19]

30 minutes ago, NCchris said:

 So, paint it or leave it black?

Leave it black so repairs are easy later on if/when you do get a chip thru the coating.

[AydinZ71 16]

I say leave it black. If you paint it while the asphalt (that’s what 3M undercoating is) is curing, the paint should stick pretty good. However, the whole purpose of the undercoating is as a sacrificial surface which is intended to be scuffed. Good move applying sealer first. I did the same, 2-component epoxy DTM sealer. Now you can spot the areas that will need repair in the future (I used white). 
 

On other areas less critical than the floor pans (above the fuel tank/cell) I used self-etch primer and applied the 3M material right on top. 

 

the other thing to consider is, black results in a higher radiative heat coefficient than white. For us who live in warm climates, this is a plus. The bottom of the car will radiate heat from the cabin. Best case scenario is a white car, with completely black undercoating. Doesn’t make a huge difference, but every bit helps.

 

once cured, you can take a piece of steel to the surface and you will notice how much more resilient it is than paint (which chips). It will fade, but not break off in chunks how paint does. Make sure you don’t apply it too thick. If it goes on in globs, it is prone to chipping. 

[rossman 19]

7 hours ago, AydinZ71 said:

black results in a higher radiative heat coefficient than white

This is going way OT but since you brought it up....black also absorbs radiated heat really well. This would cause the reverse effect when rolling over sun soaked super hot back-top and when the black undercoating is near your exhaust. 

Edited June 10 by rossman

[AydinZ71 16]

5 hours ago, rossman said:

This is going way OT but since you brought it up....black also absorbs radiated heat really well. This would cause the reverse effect when rolling over sun soaked super hot back-top and when the black undercoating is near your exhaust. 

true!!! Yeah... double edged sword. When driving though, the black would aid convection cooling as air flows over the surface, as the surface will radiate heat to the air stream. My mind is in “race car”, so no AC. I’m assuming the inside of my car will be warmer than the free flowing air around it. 
 

in general, this is going to have a really really minor effect. The body color makes the big difference since this effect how well the surface will absorb or reflect solar radiation. 
 

still fun to geek over.