We are repainting a number of rooms in our home, do we need to put primer over the existing paint?
Are there any other gotchas that we need to prepare for before repainting the walls?
No, in general, you don't need to prime existing paint, but there are exceptions.
To prepare for repainting:
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.
If you want a really nice job, get a drywall pole sander and give the wall a quick going over with 200-250 grit paper before priming, and before your final colour coat.
Remember to wipe with a damp rag (use the pole sander if you like) before applying any paint/primer.
Priming is best. If you have to do two coats of paint to cover the old color, why not use a good primer or primer/sealer and one coat of a good paint? Most people will say otherwise, but primer will stick to old paint a lot better than new paint will.
I recommend wiping the walls down with a damp cloth first, but it's a lot of work, and I've never done it to a wall that wasnt't obviously filthy.
Use one coat of primer and one coat of paint.
Never ask a paint to do a primer's job!
There's one tip that was completely missed, and should have been at the start:
When painting over a surface that has had to be repaired or was very dirty (scrub it clean first), get both the paint and the primer tinted to the same color. As long as you buy the paint/primer in the same brand and they are both the same base, you will have exact match.
This one tiny little tip guarantees a perfect job with only one coat of primer and one coat of paint.
Ok. What I've found when transitioning from one wall color to another is not tinting the primer to the top coat but rather tinting the primer to neutral gray. I've done it both ways and, for some reason, (probably what parts of the spectrum gets reflected, what gets absorbed) the neutral gray (photographers among us, think 18% gray card...) allows fewer top coats. It seems counter-intuitive but, trust me, it works!
At Home Depot I always have them tint the primer the same color as the paint. They do that at no cost. Also after caulking or drywall repair is dry I paint those areas with one coat of paint or primer before starting on trim cut in work . When I paint body .It is dry. The paint is always blotchy if those areas haven't had an extra coat.
Personal experience about dark walls.
I painted a light yellow over a dark blue wall. I used Behr paint with primer added. After two coats the blue still showed through. Not sure if prior priming was necessary since paint contained it already. One of the walls was already white and one coat covered it up.
The third coat covered it up the last bits but what a pain.