I stripped all the tiles and plaster from the walls in my bathroom and now want to renew the plaster on the two external walls (they will eventually be tiled full height), but now there are gaps to the outside world around the replacement PVC window frames. Should I fill these with mortar first or can I simply make good with the browning coat? Likewise where the sill used to be, I removed the wooden sill along with a big chunk of mortar, so I suppose my real question is: Is it OK to use browning to fill large areas and deep holes, or should it just be considered an undercoat and all making good done with mortar? I’ll be grateful for some guidance here.
First question is what weight per sqm tiles are you planning to use?
You can't put more than 20kg per sq m on plaster or skimmed plasterboard - thats without the addy and grout which will add 2.5kg per sq m to your tile weight. Unskimmed plasterboard (12.5mm) can take 32 kg per sq m.
So if its heavy tiles then it should be plasterboard.
In any event if you are back to brick and wish to proceed at a reasonable rate then I'd D&D 12.5mm plasterboard onto the brick. You can use the plasterboard to accurately form an easily tiled window reveal - yes fill round the window first but if its open to the outside then don't use plaster. Mortar the window back in.
When it comes to where the cill was use your plasterboard to create the correct height for the tiling then backfill with concrete to build up the depth - but not right to the top! Leave enough to be able to add a further layer of plasterboard on top of the concrete for tiling.
Using this method you can produce your reveal plus replacement cill exactly the right (and the same) distance from the internal edges of the UPVC window so that when the depth of a layer of tiles are added youget a nice balanced look to it.
Up to you but bear in mind your tile/addy/grout weight per sq metre.
I had a row with my tame spread just recently (can't plaster large areas for toffee me - just can't hack it). Ceiling drylined and skimmed. Walls drylined and he wanted to skim. I said no you don't! Due to tile chosen by client my tile/addy/grout weight will be about 23 kg per sq metre. I'm not having it all fall off just because your professional opinion is that a skim provides a better surface for tiling.