The way I tackle plastering for electricians - others may disagree
I assume the wires are in the tubes, and everything is reasonably rigid,
1) if at the deepest its more than say 1 inch deep, do sand cement 'browning') (1 cement, 3 or 4 of smooth pit 'bricklayers' sand )
to consistency of an icecream, adding a little washing-up liquid during mixing to aid adhesion. Wet wall in channel with hairy paint brush.
Apply with filler blade or wall paper scraper to depth of few mm to half an inch from final surface. Scratch with old dining fork, and go to bed, then treat as method '2' tommorow.
2) Start here if less than 1 inch deep, mix up "one coat plaster" adding dust to water, mix to consistency of cake mixture. This is probably stiffer than you might think, the mixture should be capable of standing un-aided in little hillocks on a mortar board. Wt the wall with a brush, fill flush with wall on either side of damage using a piece of wood as a leveling batton (keep batton wet in bucket of water.)
Once plaster is part dry, a skilled worker can skim over with very wet batton or a proper metal float to give a 'mirror' finish and allow to dry.
If you are tiling or even wallpapering over it then the un-mirrored finish will be absolutely fine, if painting, then some making good may still be required. For small areas to be painted and on show you can still omit this step, and just sand level afterwards, but this makes a lot of dust, and the pros will laugh.
If the layer is really thin, say les than 1/4 inch, then a better finish, but rather slower drying may be achieved with top-coat plaster. I'm not good enough to be able to produce the 'ready to paint' finish in one go though even with this, though it is less rough than the one coat.
Try some where un-obtrusive first, and only mix as much as you can use in a few minutes at a time.
regards M.
PS big bags of plaster dust are quite cheap compared with polyfilla or whatever trade name filler.
