Get a small board the same size as the whole (anything will work) and then stick it in the whole, turn it so it blocks the back of the whole, then put 2 screws into it, one on each side of the whole, then puddy. Should give it enough backing to stay put for the sanding.
And the important thing is to NOT have to sand much afterwards
So in short, stick something in the whole befor you puddy it, the stronger the better (hence the screws)