Your painter will skim coat to repair and smooth those hacking.
Put undercoat and repaint with colour that you need.

How do you mean the electricians didn't skim coat? Did they leave the hacks and holes just exposed? Or did they cover it up at least with some putty?
If it's deeper hacks and they just leave the hole there, they're terrible electricians. It's more complicated and takes more time, but you need to fill it up with putty yourself. I suggest the putty powder for self mixing that you can find in hardware shops. In my experience they shrink less and absorb less than the ready made putty that comes in plastic containers.
To mix the putty is easy really. You have your painting tray, and a scraping tool I assume since you need those for painting. Just put a good amount of powder in and mix water, up to the consistency that you're happy with. Most people stop when it becomes something like a thick icing. Use your scraping tool and push the putty in. You don't have to fill it up first time, just a thin layer will do. The reason being it'll take very long to dry if it's thick and won't be as strong as many thin layers. Electrical holes, I think 2-3 layers will do it. That's around 1+1+1 days drying time to be sure. If you're OCD like me, just leave them for a week after the 3 days to make sure they're completely dry.
Now the skim coat layer. Preparing it is the same, you'll use the same powder mix. No. 1 thing to remember is inspect the surrounding walls texture first. You want it to match the walls, not absolute flatness. Usually the walls will be done with a rough trowel, the texture is quite unique and very hard to replicate unless you're experienced. The best you can do is prepare a damp sponge. Use your scraping tool and spread it like butter, making a few passes. Check the texture when it's flat. Now use your sponge making light passes to try to replicate the roughness as much as you can. Don't overthink it, it'll still be visible no matter what so just do your best. It'll look less obvious after your wall is painted. Also, there's a practical use to leaving it slightly showing. Electricians will know where the wire is next time you do any renovations or replacing wires.
When that is done, let it dry for a day or so. Then you can start painting. If the area that has been puttied is subjected to high traffic like wall shelves or something, use sugar soap to wipe it down. This is an OCD step truth be told, sometimes there can be grease ingrained that will show through the paintwork. Next is the undercoat. I assume you're using water based, so planning and timing is crucial. Try to paint when no rain is forecast or later in the morning, humidity can absolutely destroy paintwork. Undercoat down, let it dry for 2-4 hours. Then on your discretion, maybe another layer of undercoat. Check when the first layer is dry, try to gauge if the putty has absorbed enough and will stop absorbing more. If not too sure, just put another layer of undercoat. Then 1-2 layers of paint on your discretion. Just remember to stop and observe after each drying period, if the color more or less has matched the existing wall in depth.

A bit long to read, sorry. Here's the important points:
1. Putty texture should match surrounding wall, not achieve perfect flatness.
2. Putty absorbs paint, so match layers of undercoat with how thick the putty is. Can't really tell, this is on your own observation.

Have fun.

Patch back the chases with cement and sand. Then make good the surface with putty.
Then follow by under 1 layer under coat and 3 layer finishing paint.
After the putty dry, paint 1 layer undercoat, wait it dry, then paint 1 layer finishing coat, wait it dry again. Then paint the whole wall with 2 finishing coat.

How do you mean the electricians didn't skim coat? Did they leave the hacks and holes just exposed? Or did they cover it up at least with some putty?
If it's deeper hacks and they just leave the hole there, they're terrible electricians. It's more complicated and takes more time, but you need to fill it up with putty yourself. I suggest the putty powder for self mixing that you can find in hardware shops. In my experience they shrink less and absorb less than the ready made putty that comes in plastic containers.
To mix the putty is easy really. You have your painting tray, and a scraping tool I assume since you need those for painting. Just put a good amount of powder in and mix water, up to the consistency that you're happy with. Most people stop when it becomes something like a thick icing. Use your scraping tool and push the putty in. You don't have to fill it up first time, just a thin layer will do. The reason being it'll take very long to dry if it's thick and won't be as strong as many thin layers. Electrical holes, I think 2-3 layers will do it. That's around 1+1+1 days drying time to be sure. If you're OCD like me, just leave them for a week after the 3 days to make sure they're completely dry.
Now the skim coat layer. Preparing it is the same, you'll use the same powder mix. No. 1 thing to remember is inspect the surrounding walls texture first. You want it to match the walls, not absolute flatness. Usually the walls will be done with a rough trowel, the texture is quite unique and very hard to replicate unless you're experienced. The best you can do is prepare a damp sponge. Use your scraping tool and spread it like butter, making a few passes. Check the texture when it's flat. Now use your sponge making light passes to try to replicate the roughness as much as you can. Don't overthink it, it'll still be visible no matter what so just do your best. It'll look less obvious after your wall is painted. Also, there's a practical use to leaving it slightly showing. Electricians will know where the wire is next time you do any renovations or replacing wires.
When that is done, let it dry for a day or so. Then you can start painting. If the area that has been puttied is subjected to high traffic like wall shelves or something, use sugar soap to wipe it down. This is an OCD step truth be told, sometimes there can be grease ingrained that will show through the paintwork. Next is the undercoat. I assume you're using water based, so planning and timing is crucial. Try to paint when no rain is forecast or later in the morning, humidity can absolutely destroy paintwork. Undercoat down, let it dry for 2-4 hours. Then on your discretion, maybe another layer of undercoat. Check when the first layer is dry, try to gauge if the putty has absorbed enough and will stop absorbing more. If not too sure, just put another layer of undercoat. Then 1-2 layers of paint on your discretion. Just remember to stop and observe after each drying period, if the color more or less has matched the existing wall in depth.

A bit long to read, sorry. Here's the important points:
1. Putty texture should match surrounding wall, not achieve perfect flatness.
2. Putty absorbs paint, so match layers of undercoat with how thick the putty is. Can't really tell, this is on your own observation.

Have fun.

Patch back the chases with cement and sand. Then make good the surface with putty.
Then follow by under 1 layer under coat and 3 layer finishing paint.
After the putty dry, paint 1 layer undercoat, wait it dry, then paint 1 layer finishing coat, wait it dry again. Then paint the whole wall with 2 finishing coat.