I was wanting to know can you build up tile adhesive to sort out a problem with a shower tray that hasn't been fitted Square ?? At the furthest away point from the wall it's about 12 and at the closest point it's about 8mm , I was thinking of using a 12mm trowel and tapering it out till next to nothing , letting it sit a few hours then smoothing it out , also was thinking about over boarding with more plasterboard , ( as in this country every backer board has to be ordered in ) but then this means doing the whole wall , and changing architrave thickness !