Glue and vapor permeability

[dikman]

After reading all this my brains hurt.........

[Beehive]

Hey, I gotta glue here in a bit. Weldwood glue. 

Takes two coats 15minutes apart. Roll the edge and leave it alone until tomorrow. Trim, groove, stitch. 

If you store the small can of Weldwood inside a plastic peanut butter jar. It'll last longer. If 3M weatherstrip glue wasn't Yellow. 

 

3M #90 spray adhesive? You bet it'll do.

[Eelco]

Thinking more about it, I think the most practical method would be a premade thin glue sheet, with tiny say 0.1mm holes punched through in a dense pattern. You could set up a simple desktop laser cutter to do that in the absence of an off the shelf product. PVB would be a nice base material since you could either melt it on with a heatgun or mist on some ethanol to activate it (not many other materials that are top-tier adhesives, match the mechanical properties of leather, can be hot melted and are soluable in a bunch of eco-friendly solvents; but not in water; infact I think only PVB fits the bill).

If you get the hole pattern right it should add minimal moisture resistance to the total of your shoe. The total effective extra length that the moisture needs to travel through the leather should be on the order of the hole spacing; a few ten's of a mm or so. 

It should easily be strong enough to bond linings to outers; and maybe also leather outsoles; but again if you add a rim of a proven shoe cement on the outside it should be totally fine I imagine.