Kent:
I'm pretty sure that is just overpriced undercoating. Of course I have not put a screen door in a boat to test my theory, but all of the advertised properties are the same as a quality undercoating.
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-- Paul --
I'm with Paul.
I believe it falls into the "consumers will fall for it because they want to patch something instead of making a proper repair" category.
If it is a rubberized product that is supposed to form a "membrane" of sorts with the "rubber content" that is good, as waterproofing is the goal, but remember that paint, or as Paul suggests, rubberized undercoating does the same thing, that is, forms a skin or membrane to prevent the intrusion of water.
Many so called "roof sealer, or gutter sealer" products in a spray can also contain an agent that causes the product to "foam" or expand, allowing the product a better chance to get into small holes or cracks, stopping leaks sometimes missed by a quick one coat application of a similar non expanding material.
It is probably a great product, but as with shopping at a 7-11 you pay for the convenience.
Probably get a comparable product at a farm store, or automotive store for much less.
One can seal a screen/screen door with plain old paint too...
If you do a quick Web search for "Flex seal review" there are a lot of pissed off people commenting...
Rob
Waiting for "someday" will leave you on your deathbed wondering why you didn't just rearrange your priorities and enjoy the time you had, instead of waiting for a "better" time to come along...