It comes in a spray can, and is essentially asphalt with thinners for spraying. I have already used it on the rear wheel wells, but I have a chance to switch it up on the front. I’m ready to re-spray the front chassis-side wheel wells (which are huge on a 240z) with epoxy and I still have a lot of the 3M asphalt on hand to spray within the cute window. I searched the forums but did not find any discussion about whether it is garbage or reasons to not use it. I suppose the alternative would be a 2X urethane undercoating.

Looking forward to your thoughts! I was planning on spraying tonight.

I went ahead and applied the 3M undercoating to the DTM epoxy on day 1 of 3 (epoxy cure) . I used the feathering technique Chris recommended. I left a 1/2" overlap of asphalt over the adjacent clear-coat. I sanded the urethane/epoxy mask line so there would be a nice transition that is unlikely to lift. fingers crossed.