I have knocked a (too large) hole through a single skin wall for a catflap, but need advice on making good and lining the tunnel. Wall construction is (from outside) render, brick, polystyrene (insulation), plasterboard, plaster. I have supplies of mortar and ready mix plaster. How should I line the tunnel? Can I use mortar for the length of the tunnel with a layer of skim on top? I am concerned that if I do a base coat of mortar that it will not bond to the polystyrene or the plasterboard. I will fit a flap on the outside too - can I use ready mix mortar to patch the blown render on the outside around the tunnel? And will the fresh mortar be drillable (to attach the catflap on the outside)? Many thanks.
