Filler that shrinks is usually the deep hole
I filled a deep hole behind the door on a Saturday morning, the scar where a shelf bracket had pulled out. I packed it in one go because I wanted to paint in the afternoon. By Sunday the face had sunk. A crater, a hairline around the edge, and a plug that sounded hollow when I tapped it. I filled the crater. It sank again. The filler was fine. The hole was too deep for one pass.
Filler is a paste that dries hard and can be sanded. It is not a brick. In a shallow dent it does the job. In a hole you can lose a finger in, the middle stays wet while the face skins. Then the middle shrinks, the face goes with it, and you have a dish you will see under emulsion. People blame the tin. The tin was doing a skim's job in a cavity.
I now pack in layers. I brush the dust out first. Loose grit is a hole that will move. I wet the plaster a little if it is chalk dry, not so much that I have a puddle. Then a first pass that keys into the sides and does not try to finish flush. I leave it. When it has gone off, I put the next layer on. The last layer is the skim I can sand. If I can still press a nail into the face, it is not ready. I have painted that. The shine shows every sink.
Depth is the check. If the hole is only a rawl-plug scar, one pass is enough. If I can see the block, or the void behind the plasterboard, I am not filling that in one sitting. A very deep void wants a backing, a scrap of board or a ball of newspaper packed in so the filler has something to sit on. I would not try to make a stick of filler stand in air. It will slump before it shrinks.
Summer with the windows open dries the face fast. That is when I rush. The skin goes hard, the core is still wet, and the crack appears the next morning. Winter with the radiators on dries the air and pulls the moisture out of the same face. Same crater, colder room. I would rather do two thin passes on a Tuesday and a Wednesday than one lump and a reskim on Sunday. I work when I can sand without heating the room to a kiln or leaving a draught on wet filler.
Skip filling over wallpaper dust and calling it keyed. Skip a wet dollop you smooth until it looks painted. You have sealed a wet core. Skip powder filler mixed to soup because it spreads easier. Soup shrinks more. Mix it stiff enough to stay on the knife. Skip "one coat" on a hole that still takes the blade to the hilt. Skip sanding after an hour because the face feels dry. The face always feels dry first.
I also look at why the hole is there. A rawl plug that pulled out will pull out again if I fill and then drive the same plug into the same crumb. I move the fixing, or I use a plug that can grip what I have left. Filler is not a wall plug. I have treated it as one. The shelf came down again.
This is not a paint catalogue. I do not need five tins. I need the hole less deep than my patience, and layers that have actually gone off. A small tub I will finish beats a bucket that skins in the pot.
I do not need a different filler because the last hole sank. I need to stop filling a cavity in one go.
If I later add a buy link here, it will be a paid promo.