I've installed new bathroom light, since closest power source was across th bathroom (no access above/bellow) I had to "dig-in" the cable into the wall.

House is about 40 years old so wall isn't drywall but.. I can't remember the word for it, it's like cement.

Anyway, I plan to put the cable in, plaster it using drywall compound, sand, repeat etc. and then use primer before painting everything.

Any tips on how to avoid this patch being visible later?

Cable runs up one wall, over ceiling, and then down another wall, I dag a small area, maybe 3-4mm wide and 5mm deep.

Thanks!

I just recently re decorated my living room, our house was built in the 40s so all walls are plaster. If you sand well, primer paint, you won't have any problem. If you are unsure, have a professional do it for you.

Tick

Sounds like you have plaster walls and ceiling. You really shouldn't bury electrical cable in the plaster. Even though you have plaster there should be a cavity in the walls and certainly will be one above the ceiling. You should go through the plaster to the cavity an pull you cable there. Whether in the cavity or buried in the plaster you cable should be armored cable (AC), commonly referred to BX or metal clad (MC) cable. You don't want to be nailing or screwing into your cable later on.

Look for a product called "ShurPatch"... pack it into the crack or hole and wipe it off smooth( cause it doesn't sand easy) Once that dries , top coat with normal drywall compound and sand flush, prime then paint!