Ye Olde Inn

Im in the process of stripping all my mini's back to plactic (to the people that suggested Detol, I cant thank you enough).
Im planning on using a spray primer on all the minis but wondered what colour would be the best starting point and if its better to use different primers on different mini's ie:

Chaos Warriors - Black
Heros - White
Fimir - Black... and so on...

What are your thoughts?

Aren't primers primarily depending on the colors you wish to paint on top of it? If you want a light color scheme, you should have a light primer, and for a dark scheme, a dark primer. But I admit that I haven't painted minifigs in ages.

I am not a professional painter but I've seen that even Mike McVey (Citadel official figures painter) was using a white primer. You can read the basic of the painting on the back of the UK Questbook. The 3 figures you can see there (Fimir, Gargoyle and Barbarian) are painted by Mike.
In the past I've primed my figures with black primer and I have to say it was very hard to make some colors stand out, like yellow, dark green, orange or some reds too.
Also on White Dwarf 113 there is a smal guide. I attach you a page here (on the left column, behind the Orc picture):

i use white on everything these days, as i work with a lot more wash type style of painting. this doesnt really work with a black undercoat. as it takes more coats of paint to build up a colour over black.
best thing i can suggest is try both.
i dont use any expensive spray on primers either. i just get mine from halfords for like 7 quid.

a lot of this is dependant on your skill level of painting too. if you are starting out. i suggest going with white and making sure your paints are thinned down a tad with a little water and building up your colours with multiple layers.

My usual go-to primer is GW's Chaos Black (and this has been my favourite option for many years). That being said, I painted virtually all of my HeroQuest models over a brown undercoat (GW Mournfang Brown spraypaint) and can only recommend this option, because it's dark enough to make for relatively easy painting on top, yet also leads to slightly more vibrant colours than a black undercoat would. So for "conventional" painting techniques, I'd recommend a brown undercoat for your HQ models.

For a more wash-oriented style (or for using something like GW's recently released Contrast Paints), a white (or very light, in any case) undercoat definitely has its strong points.

I use black for almost everything, it hides a multitude of sins and adds some easy shading. I use white only for areas that will have the lightest paints (white, yellow, pale flesh) since otherwise you have to do several coats of white over the black.

Grey then white from above is my fav, shows you instantly where to highlight

So here is my question (figured I’d tag along this thread):

Is it absolutely necessary to remove the old paint job or can you just primer over it?

I saw a video recently by someone who was explaining the stuff he wished he'd known when he started painting. Using a wet palette, and for primer … he uses both!

He primes in black, then dusts the top with white. This gives him a strong start for washing and highlighting both.

I've also seen someone using a white primer first and then a brown or black wash to the whole miniature.
Also there was a (lost) blog of a guy painting a Minotaur and explaining some sot of "undercoat" technique which consisted of paintinting the whole miniature in black/white first and then applying base colors over the already shaded piece, so you don't have to apply shades to all the colors because they are already on the white undercoat. Unfortunately I can not find his site anymore since a lot of years.