thanks, Steve!
Did some experiments yesterday.
After about 6layers brushed on the mold it was strong enough without backing. You can speed up drying with a hair drier
It will take ages for a finished seal, but it is worth it. and I have to use a new brush or every single layer.
Enforcing with fabric after 2 layers gives a strong skin, but you have to be careful not to trap airbubbles. With all the edges of the ribs, this will be complicated and frustrating.
In the meantime, I work on the "master" rib which will be duplicated for the first proto.
Thanks for all your encouragement!