What your talking about is 'true-flames'?
Like this;
As in a more realistic looking painted fire compared to a cartoony 'hot-rod' stylized flame?
Like this;
(Images shamelessly ripped off google search, may not be the best examples out there,but they illuistrate the point)
Thats what we call them in the custom auto paint industry anyway, There are bought stencils, you can print the shapes and cut them out I suppose, its not easy peasy to pull off, but its not impossible, it takes practise to get used to how to use your stencils to make the effect. You will need transparent, or candy colours,red, orange, yellow (or other for coloured flames?) and unless the base colour is white, some opaque white too.
Also this technique requires an airbrush, preferably a fine detail one.
example stencils;
The general jist of it is to draw a curvy line with the airrbush in opaque white, and then use the funny shaped stencils to find bits of curves that match with your line and beef up the lick of flame that way, all in white, do a few, not too many, and candy over the top all one colour, then put more licks of fire in with white, and candy again, rinse and repeat, stepping up through the colours untill you have your realistic fire.
It takes a long time, works best on flatter open surfaces. Very popular on stuff like motor cycle tanks.
The main issue i can see is the scale, warhammer stuff is small, and this effect is tough to do at a large scale anyways. Never the less, could be done
if done well, could be really amazing to see.
For the stencil material, something, I use acetate sheets most of the time, for a more durable one (which is more of a paint to cut out) I use laminating pouches, thick ones.
For a one use stencil paper is good enough, card maybe a few uses, but plastic sheet ones will last the longest.
It also depends what kind of paints you plan to spray at it. Acrylics will hardly eat anything, any alcohol or better will maybe do something... hard to say.
