I read a great quote from an old painter once who reckoned when it came to painting preparation people either did not enough or TOO MUCH!
He said if you were puffing while sanding, you were sanding too hard. The idea is to give your surface a ‘key’ or vey light scratching that the paint can stick to. We are not trying to sand old paint completely off.
For interior painting trim such as doors, windows, skirting etc. the same applies. Paint manufacturers often advise that old painted timber should be stripped back to bare timber! Well, I suppose anything is possible – if you want to pay for it. A good scrape, fill, sand, wash and wipe is really all that is needed.
Similarly good solid walls and ceilings may only need a quick sweep to remove dust or cobwebs. Its not usually necessary to undercoat the whole area.
Surfaces to be painted need to be clean and smooth with a ‘key’, but stripping off ALL previous evidence of paint is just a waste of my time and your money.
I've been drawing and painting since I was a kid. I've studied art and design and have worked in visual areas my whole life. Now I'm a house painter!