Jeff:
Never have had any real success w/ rollers - and besides, rollers cannot likely really emulate how zimmerit was mostly patterned...
I use drywall compound mixed with latex paint - dies like fine plaster in grain, but more resilient and tougher. Nicely chippable and spalls like the real thing did. Adheres reasonably well to both the kit plastic and to any applied sub-zimm paint... And can be safely totally removed with water if desired.
Tooling DIY zimm: Real zimmerit was applied as a paste layer, and stamped (such as to create those "waffle" patterns) with a wooden block or troweled on to create the "classic" ledge-and-trough patterns. It was not rolled on. So, do just what real zimm'r folks did! A small stamp "die" will create the blocked patterns, and small trowels / knife-tips / screw-driver blade tips / pieces of scrap PE all work to score for the ledge + trough patterns.
I think that one of the "secrets" to getting good-looking zimm is to carefully examine photos of the real thing... get a good sense of the relative scale and appearance of the coat relative to various "known and constant" hull and turret-features (the hull MG and vision-ports are good landmark features, as are tools and OVM...), and note that the patterns are never actually entirely orderly nor really straight and "machine-like" in their apparent repetition. These irregularities in the patterns, together with "proper scaling" will make your coat much more realistic-looking.
So: Recap: I can't make rollers work well, either, so join "my" club with that frustration! Use a technique similar to the real application - apply thin layer over small area, tool that layer with tiny trowels or rakes or stamps, and move to the next area.
Trowels and rakes take practice, too, but working small areas and being patient and taking your time will be worth it. Learning the best angle to press in a tiny trowel is pretty easy - just experiment a little until you get a trough and ledge that you think looks right!
There is NO instant Good Job nor any really quick, simple, no-real-effort way to get good-looking 1/35 zimm... You are trying to tool on a fairly complex surface which has definite appearances. Most DIY zimm jobs with most putties and such look very un-realistic, IMHO, zimm decals and zimm PE likewise. Zimm resin looks OK generally (because someone actually tooled up the master right!) if applied right, but it has its tricks...
Give trowels and such a try!
Bob