you need to cut back the hole to the nearest joist (halfway on both sides) then insert a piece of plaster board cut to the hole size, scrim tape over, then plaster over,the scrim tape,
Polyfilla you ain't got NO CHANCE.
wirecutterman, December 2009
Polyfilla!
Roy Williams, December 2009
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I can answer this question...
How to repair 6" hole left by removing ceiling fan?