How do I prepare an already painted wall for new paint?
No, in general, you don't need to prime existing paint, but there are exceptions.

To prepare for repainting:


wash the wall to remove any grease.
fill any holes and cracks with suitable filler, possibly using flexible filler for longer cracks. Unless you're an expert you won't get the filler flush with the existing wall so you'll need a filler you can sand.
sand and prime the filled areas - priming will seal the filler and keep it in place. If there are a lot of filled areas, you might want to sand and possibly prime the whole wall to avoid a patchy look.
repaint the wall with at least two coats of your new colour.


If the cracks are large you might want to check there's not a current movement problem. New houses will settle quite a bit when first built and there'll even be some movement in older homes. If there is current movement then you'll be seeing new cracks appear and older cracks getting longer and/or wider.

If you are painting over a dark colour with a light colour you might need more than 2 coats to stop the previous colour showing through. If that's the case then using a primer or basic white matt emulsion for the first coat (or two if it's really dark) is a more cost effective option than using the more expensive coloured paint for all the coats. Unfortunately you don't always find that out in time.

Paint in full daylight - especially for the final coat - so you can see where you've been and be methodical.