How to cut a channel and plaster television cable work in a brick chimney

In the UK, standard practice is to coat walls in your situation with a mixture of UniBond PVA Adhesive & Sealer and water (instructions on the tin will give recommended mixing percentages).

Then paint over everything with either:

• A watered down white wall emulsion mixture (again recommended mixing percentages will be found on the tin).
• A white "block out" wall emulsion.

Then paint your walls with your chosen wall emulsion colour. In your case, as it looks like it's a bathroom of some sort, I would go with a emulsion paint specifically for high moisture areas (such as bathrooms & kitchens). Doing so will have the benefit of lasting longer in those kind of conditions, also those specific type paints offer a lot better protection from mold forming on walls in high moisture areas.

Just throwing out a (crazy) idea you might also be interested in, seeing as you are leaving areas of exposed brickwork (not plastered over).

• Clean the exposed brickwork thoroughly, then seal that brickwork with an appropriate clear (waterproof) sealer.
• Then paint the plastered areas with your chosen colour (as per above recommendations).

The contrast between the exposed (sealed) brickwork and painted plastered walls can make for a great effect (if you like that kind of thing).

I won’t say I'm an expert; I've worked with someone doing something similar in the past and may at least give you some guidance.

Personally, I would use a concrete saw (larger blade than an angle grinder) and/or a chisel to cut the opening. Make sure you have help as you DO NOT want to drop the removed section(s). You can use these bricks later (see below).

Don't forget the header. I know I'm using the wrong term, but I'm referring to the steel plate that holds up the brick over the window.

I would then use a chisel to remove the halved bricks and either turn them around or replace them with good ones from the removed section.

I would also recommend that you at least talk with a mason. They may be able to replicate the detail work around the window with the bricks from the removed section.