Lots of great replies so I'll add my small experience:
Oil paint over
totally dry water is fine and vice versa - at least 2 days drying for water; double for oil - that's my method (usually coz I've made a mistake tbh and got the colour wrong / decided I wanted a more durable finish) you might get just as good results in a shorter timespan; but frankly I'd rather wait than have to strip it and do over again.
Never tried anything over milkpaint, but from what I've seen it can have very low adhesion - flakes and cracks on it's own without help, which could be to your advantage depending on the look you are after.
Wax over any of the above is fine, at least after 24 hours drying - the few bits I've done beeswax over oil and water have turned out great and silky smooth - lovely.
As far as colours - what over what - it all depends on what look you are going for: subtle blending whereby it's more modern white over ivory (simulated old yellowed "white") and only lightly cut back in a few places to show wood, or trying to replicate the "I grabbed the closest tin of whatever paint and slapped it on" which means there will be more aggressive cutting back in more places and to the wood; hence the next colour being slapped on to "freshen up".
The single colour family trick can be made more realistic by doing a paint chipping effect we use in scale modelling whereby the base coat has places of wear and or corners covered with various things like wax mushed up (stippled or whatever) with an old crap short bristled brush or wet rock salt (a good one as it creates a very random effect), as liberal or sparing as you like then allowed to dry. Paint over all of it again, then clean off areas of salt by gently rubbing them out but try to leave the outline as best as you can. The wax you heat with a heatgun or something gently and wipe off, and if you don't get it all off that's fine too as the next layer won't stick or have defects that adds to the effect - another alternative is dabbing paint remover randomly letting it cut to the previous or more layers then removed by wiping and neutralising, plus damaging the piece itself with a file or something around exposed edges Knocked about / dog / cat scratches) ; best done with 3 or more coats with treatments done randomly in different areas each time - great to fake age even on a single colour item.
The reason for the above is when you put the next coat on you get the visual effect of dimpled cratered paint, whereby even if you paint over that section with colour and don't cut it back, you can still see that there has been more layers of paint in the past.
If you really wanted to go for it you could deliberately introduce paint reactions in random spots to get crackling, bubbling effects - not just crackleglaze which to my eye is a bit too obvious if done poorly - but more like replicating mistakes people get when they have no idea what they are doing - which is usually the case with the "slap it on, whatever colour it is" favored of utilitarian country folk prettying up something for the misses. A way I've had this in the past (totally by accident) is using volatile solvent based paint like nitrocellulose or xylene (a serious headbanger - potent stuff and utterly ruined 4 days work!!) over oil or water - such as car spray paint, or the thinners for the effect, it will react and cause serious crazing or bubbling which you can leave on or remove - but NOT the acrylic car spray paint versions which will go over oil and waterbased with no little to reaction effects at all (acrylic car varnish is great for a fast satin finish in a pinch, but hard to get even on a large piece, best used for small stuff) - but oddly I've just remembered oil over acrylic spray paint WILL give subtle crazing - a wood window cill I was trying to finish fast at home - sprayed with (
*embarrassed*) white fridge spray paint (it was leftovers and a :!: bright idea :!
- didn't cover or work very well so I painted over with oil gloss when it seemed dry but obviously wasn't - looks ok from normal distance, but if you look closely there is crazing (differing drying speeds which is how crackle effect occurs) - some dark wax or thin paint wash would bring this out.
Depending on how many coats of paint you want, the last one from the top could be applied rough style with a coarse brush (worse the better like a masonry brush) to introduce heavy brushmarks, then the last coat will subdue them a bit.
If there's carved detail you could make a small pre mix of the paint colour with talc powder (not flour, it'll rot) to thicken it, then paint over the detail first and let dry, before the slap coat so it's less defined - again like it's got a dozen or more coats on it as you see with plasterwork in old houses.
The "Slap it on" method should use contrasting paints so the cut back area's are as obvious as possible and you'll need to be more liberal with them too, it's hard used unloved furniture and should look like it.
Then there's the "recent 70's owner who likes it natural but is unskilled" method : a plain wood piece that got stripped with a blowtorch and has a few light scorchmarks here and there, maybe a heavier one or two roughly sanded out ish - if there's detail areas, there's bits of paint left in crevices - a good way to fake old thick paint here is some powder colour mixed with thin watery superglue and dabbled in - it will set in a minute or two, so make only a small bit at a time and have realistic thickness (but sets like concrete so beware !!) - waterbased might be ok to mix, oil... dunno not tried it.
1 obvious mismatched handle where there's a few - odd bun foot, square lump on plainer items, mismatched wood ...
There's many more of course, but that's a sample of personal experience and tips I picked up from TV / books etc... lol blimey I actually look like I know what I'm talking about ...(gizza job?)
