You want to fill the gaping holes around where a new light switch has been fitted.
You will need
Step by step process
Ensure the surface and gap is dry and free from loose material.
Using a filling knife, apply the filler directly into the gap - then smooth off with a wet knife. For holes or gaps over 20mm deep, build up in layers and allow to dry in between.
Once dry, sand with fine sandpaper for a perfect finish ready for painting.
Can be overpainted with care once surface is dry. After use replace lid, wash tools in warm water.