Hi, from what you say the problem may be the paint you are putting on them. It sounds as though you are putting emulsion on, even dulux satin is emulsion. I would never put emulsion on them. I would use Dulux primer on bare patches then undercoat and finish off with either a gloss finish or if you want slightly duller finish then use eggshell. If you have used emulsion then sanding them can be a problem as more of it is likely to peel off unless you are very lucky.