Today I have made a water test to find out if the casein glue is waterproof.
I put a 6-7 inch long part of the hollow beam in the filled bathtub.
The results are the opposite of encouraging, the glue failed completely after three hours in the water.
The former hard drops of glue excess inside the beam have a consistency like rubber.
All four plywood sides still holds together after I picked it out of the water, but it takes just a little force to separate the pieces.
Although so much water exposure will only happen in very rare cases of daily use, I warn You to use casein glue as adhesive for weather exposed parts like a bicycle frame.
Even the water that will be sprayed from the front wheels in foul weather can result in a frame falling into pieces -> an accident -> injuries or death of the driver
Eventually a layer of GRP on the outside surface will make such a construction nearly waterproof, but then I prefer to use epoxy completely.
Pics of the failed glue joints.
The dark spots are screw holes