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At what point is filler no longer useful and the plaster needs a piece replaced?
For any hole over 10cm wide, you should consider installing a new piece of plaster @stuartheth. You can use a reinforced plaster patch for any hole between 5-10cm. Anything under 5cm, I'd be filling. A problem arises when you try to fill a hole bigger than a 20-cent piece where your filler simply falls into the hole. You can build it up with several smaller applications waiting for them to dry in between. Alternatively, you need to find a way to back the hole and prevent the filler from falling in. There are a few ways to go about doing this. The first, if the hole is large enough, is to pack the cavity full of newspaper, and then when you can't stuff anymore in, slide in a folded piece of cardboard. Once it's inside, unfold it, so the newspaper pushes it against the back of your hole. Extra points if you remove the top paper layer and expose the rippled internal section on the cardboard; that's a great bonding surface for the plaster. The other option is to glue a couple of thin strips of timber inside the hole with Liquid Nails. I've created a rendering below to illustrate.
To answer to your question, it really comes down to how solid the repair is. You don't want a large section of wall where if the area is lent on, it caves in. The better support you can add to the inside before doing the repair, the larger the hole can be before you need to use sheeting.
Please let me know if you have any questions.
Mitchell
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