There are several steps in using powder paints/earth pigments to paint rawhide:
Bottom Line: Rawhide will always be subject to warping due to humidity. Also, since rawhide is not uniformly thick it will expand and shrink accordingly. Additionally, if it is painted, the rawhide will absorb some moisture, causing it to expand in the painted areas but not in the unpainted areas.
For rawhide that is not permanently stretched in a frame, etc., there is no ready solution to be able to get it perfectly flat and for it to stay that way. It will always be warping whenever the humidity changes. (Even bringing it out of the house for several hours, then returning it can subject it to humidity changes.)
The only good partial solution to keep rawhide fairly flat is to physically wrestle with it and try to bend it over the knee, etc., in the areas that are warped the worst in an effort to get it somewhat flat.
If really FLAT rawhide that won't change shape is desired, then a fresh green hide is required. (The hip section of a buffalo or cow is desired - NOT the hump, as has been reported erroneously in the literature.) This should be a section about 3 times the size of the desired finished shield, and then it must be fleshed and de-haired.
For information on painting your rawhide shield, see Powder Paint Preperation & Application Tips.
The following items are useful in working with rawhide for various projects.