Will flex seal work for weatherproofing outside shed?

Paint is weatherproof is you get some marine paint, and you can buy an additive from Rustoleum Never Wet will repel water.... https://www.amazon.com/dp/B00HRX6N0Y/ref=asc_df_B00HRX6N0Y5174313/?tag=hyprod-20&creative=394997&creativeASIN=B00HRX6N0Y&linkCode=df0&hvadid=193184886406&hvpos=1o1&hvnetw=g&hvrand=16140057570315398884&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9033057&hvtargid=pla-307761509910 watch the video

Yes, it will, but it might be costly. You can use the spray foam that expands or the sealing tape that keeps even oxygen out. Duct tape could also do the trick. It would be best if you could insulate and seal with drywall or paneling, including the ceiling, and weatherstrip any windows and doors in the outside shed. Best wishes 😇

YES!!

I had my handyman climb up and spray EACH and EVERY screw with clear, spray Flex Seal over 2 years ago (cuz I had a leak!! aaaaaack) Works like a charm! I am a bit obsessive about it now, since a few things got wet/ruined - so I will spray inside this year before winter. Doesn't cost that much for peace of mind. (it's odd stuff in that it behaves like hot glue - leaves webs of overspray etc. - play with it before you climb up and start)

I found this article that might be helpful for you - http://doesitreallywork.org/flex-seal/ - good luck

Ha Ha! What a gimmick! Products similar to Flex Seal have been around for ages made by reputable companies like DOW and 3M, etc. And also cheaper. A good primer and outdoor paint will protect the surface. Weatherstripping the doors and windows will protect from wind, rain, and snow.