[alexjones]

Hello all,
New to the forum
 I am trying to find out if there is a soft material that will hardens like plastic(or hard rubber) when it contacts with air.
also a little bit flexible like a thin(3 or 4mm thick) plastic( or hard rubber)
I was thinking of those insulation foam that hardens when dried out but be more durable like plastic.

Thanks
Alex

[Enthalpy]

Yes, it's called a varnish. Chemical companies make varied formulations now; historically, they based on siccative oils like lineseed oil (beware slightly toxic). Siccative oils react with air (something like the cross-linking of double bonds?) to get solid.

Put successive layers to attain the desired thickness, since the drying or hardening time increases quickly with the thickness of the new layer.

The hardening time speaks against a reaction with air, which needs air to diffuse through the layer, and combines badly with 4mm. Are there good reasons against applying a molten polymer or a two-components resin? A polyester resin seems a more natural choice than any air-hardening thing.