I'd try a heat gun to warm it and scrape it off before you add a bunch of chemicals.
it is super-sticky stuff. I used some on a repair on my dory last summer. It responded to chemicals, but seemed to absorb them before releasing its sticky hold. Reduce its quantity (heat/scraping) and then try chemicals to remove the residue. I'd go Mineral spirits first since it's less toxic/penetrating than acetone.

 

Thanks, I used a heat gun to remove a couple of failing handles last night, but hadn't really thought to use it to remove the flexseal. I might give that a try. I've tried both acetone and mineral spirits. I tested both out on the thicker bottom material just to be sure they wouldn't damage the boat, and neither did. The acetone worked but it was not fun to be around and evaporated so fast that I had to keep adding it which did not help with the fumes.

For anyone else trying this I recommend using a respirator with acetone, but if you have mineral spirits, that's working pretty well actually. It doesn't dissolve the flex seal quite as fast as the acetone does, but it doesn't evaporate nearly as fast either so you can work with it a lot longer. I'll probably keep using the mineral spirits for most of the job, as it's actually going a lot faster than I thought it would. As far as I can see neither chem has caused any damage to the boat material, so I think either would be a good option for getting rid of flex seal.

 

I tried "Flex Seal" on the bottom of my of two of inflatable Kayaks and it stayed tacky even after a week of room temp drying. I then tried a heat lamp to speed up the curing process and it did the exact opposite - it mad it tackier. In researching I found that Vinyls (such as the Reinforced PVC used in inflatable kayaks) have oils that are absorbed by most adhesives - and evidently this rubber sealer - which causes it to turn gooey. But luckily I found that Mineral Spits using a rough-ish bath towel (not a t-shirt rag) removed it without too much effort and didn't appear to harm the boat's vinyl ether. Re-reading the spray can of "Flex Seal" it does warn "May not be compatible with all plastics, vinyls & rubbers." so I have no one to blame but myself.

 

I think I would grind it off

 

I bought an old Riken that had a large black rough patch on it. I'm pretty sure it was Flex Seal. It took hours to get off and properly patch. I found the best way was work in small sections, soak in acetone, then sand off with an 80 grit sanding block and repeat. As I got closer to the actual raft material I increased the grit of sand paper I was using. Mineral spirits did nothing.