What is PAX?
PAX is a mixture of prosaide (prosthetic adhesive) and acrylic paint. You can use it to paint your skin for approx 4-5 days before it starts to flake off as your skin renews itself underneath. Typically the mixture is 50/50 though you may want to experiment a bit to find what's best for your own skin. You may find that closer to 70% acrylic paint is better if you only intend to leave it on for one or two days.
Where do you get PAX?
You have to make it yourself, but the constituent parts are easy to get hold of.
The best place to get prosaid was www.henna-boy.co.uk, but they seem to have stopped selling it? You can do pretty well on ebay. You should be able to get 100ml for £20 which is way more than enough.
Craft/budget grade acrylic paint is perfectly fine and available at most art shops. I tend to get "student artist" paints just because there is more guarantee of colour consistency from batch to batch and they dry up a bit slower.
Stick an equal mix of both these things in a pot and stir it up until there's no streaks in the colour. if making white PAX, stir it a bit more, because you won't be able to tell when it's fully mixed. I find as long as the lid's decently tight, the mixed up colour will last for a couple of years or more.
How do you put it on yourself?
I tend to get the cheapest bathroom sponge I can find that doesn't have massive irregular holes in, and cut bits off it. You won't get to use the sponge more than once so cheap is good here. Dab it gently on- don't rub with the sponge, or it will look manky. You can do several thin coats so don't get too overenthusiastic on the first go.
PAX is *excellent* for scales. The magic ingredient is fishnet tights. Dab on a layer of base colour as before, and then stretch a piece of fishnet tights over the area on which you want scales, and *carefully* dab on a small amount of paint of a different colour through the fishnets. Putting the tights in an embroidery hoop (available for a couple of quid from most sewing/craft stores, or the internet) makes it a lot easier to do this without a helper.
Stopping it coming off again
PAX will be a bit sticky unless set with some kind of powder. Translucent makeup setting powder is ideal, but plain old talcum powder will do if you're on a budget or working on short notice. On most skin it'll last like this for four or five days. Powdering once a day is ideal but not essential.
Making it come off when you want it to
Proprietary prosaide removers are available. I'm told isopropyl alcohol works, but I haven't personally tried either of these things because after four days I can remove it by scrubbing with hot soapy water and a flannel. The more acrylic paint in the mix, the easier it will be to remove, but the easier it'll come off before you want it to as well. Johnson's Make-Up-Be-Gone wipes are also pretty useful as they dissolve the pax better than other similar brands for no apparent reason.
Spray-on quick drying sunscreens are your friend. I'm using Ambre Solaire UV Sport Mist (comes with a variety of marketing claims about being impossible to sweat off and letting the skin breathe), putting it on once a day over the makeup and then powdering when it's dried. Face is just about surviving Friday to Monday doing this. And it comes in factor 50. I'd normally only use factor 20-30 or so but if you have paint on some but not all of your face it's a really good idea to make sure you don't tan markings into you...
PAX in many cases is the Wonder Paint that is the answer to all your hatred of having to put on a complex makeup job anew every morning. But it does have some limitations that you may want to be aware of. I don't recommend scales without a base colour, or any pattern involving thin lines or small areas, because PAX does come off a little at the edges of each individual painted bit, so you'll lose a lot of definition.
You're also likely to find a certain amount of paint flaking off from areas that come under constant abrasion- under bra straps or circlets, say. I have colour-matched my PAX for Eleri to two colours of Diamond FX paint so I can use PAX on my face and conventional paint across my collarbones and neck, and do all her scales in facepaint for single-day player events. Others' mileages may vary.