I'm about to tile a shower alcove. However, I put a straight edge against the walls and the rear wall bows out very badly. The wall is 950mm wide and putting a straight edge across shows a gap about 8mm at it's max in the centre - i.e. big enough to get my finger through! I'm using 500mm wide tiles (by 250mm high) in a brick pattern. I did have a go at tiling it as is and just putting the tile adhesive on quite thick in the centre of the wall to try and fill out the gap a bit, but it looked awful so I've taken the tiles off.
The wall itself is a plastered external brick wall. The house is 1930s. I live in the UK.
Please could you advise what would be best to fix the issue to give a flat surface to tile on to? I was thinking of getting some ready mix plaster and spreading it on to level the gap - i.e. I'd spread it about 8mm deep in the centre and feather it out to 0mm at the edges, using a straight edge to check as I go. I wouldn't attempt this myself if the plaster would be the finished surface, but I think I can do a decent enough job given it will just be tiled over. Does that sound sensible and if so:
- what type of plaster should I use?
- do I need to prep the wall in any way before plastering? I've already painted on the primer the tile shop sold me that was to prep it in advance of applying the tile adhesive (I think this is just some sort of glorified PVA).