Is Bendable Plywood Durable Outdoors?
Flexible plywood isn't marketed for outdoor use, but here's one anecdote of the material holding up in an exterior application. April 18, 2015
Question (WOODWEB Member) :
Is it ok to use bend-a-ply for exterior use?
From contributor L:
The bending ply we get isn't water resistant. Check with your vendor.
From Contributor H
I've never seen a bending ply that was rated exterior. That being said I have personal experience that suggests it is up to the task. Over 15 years ago I built a single step up from ground level to a slightly raised porch. It needed a soft detail on the ends so I ran the tread ends and riser with about a 6" radius instead of a squared off butt end cut. I wrapped the riser with 3/8" bending luan with one edge sitting right in the dirt. It was sort of an experiment. Well that riser is still solid after all this time. It's right on my own property so I see it all the time. On the other hand I've started to shy away from 3/8" bending luan as a general rule. Last year we had a serious project failure due to delamination of the product. This was interior work so not really relevant to your question - other than to raise some concern of the product in general.
From Contributor Y
There is a plywood called bendable plywood with a single face veneer and core. I think that is what you need and it can be used for exterior.