Hi!
Your best option, IMO, is to go with a DIY zimm coat. The subject vehicle is a pretty good one for doing this, in that most of the zimm'd surfaces are flat and easier to sculpt then if you were working a turret and gun mantel.
Putty does work, but I always go a very different route...
I use the ready-mixed dry-wall compound available in small tubs at Home Depot and other home centers. I mix it with a little latex wall-paint (don't have any at home? Just pick up a free wall-paint sample from the paint dept at HD or Lowes!).
The latex paint adds some "plasticity" and robustness to the otherwise soft plaster. The plaster is fine-enough grain to nicely mimic the granularity of real zimm in 1/35th scale. The mix goes on and adheres nicely to any styrene hull without sanding or other prep, and it does NOT damage that plastic surface in any way. It is completely safe to use, dries pretty fast, and is readily sculpted with any rake or other tool that you use with putty. It's all-water clean-up. Oh, and it is CHEAPER than any putty.
There are a couple of other specific properties of this mix which, I think, make it ideal for this use: 1) Real zimm "spalls" and chips off the hull with bullet strikes. You can easily and very realistically chip and spall this model zimm. 2) Real zimm has a specific color-range. You can use any tint or paint-color and get a very realistic material color in your zimm. This means that spalls and chips look RIGHT without additional painting or coloring. 3) The stuff goes on over both styrene and paint surfaces, so you can actually do a "before zimm applied" paint layer - whether that be primer, base-color, or (and possibly well applicable to SturmTiger, which were built using mostly used Tiger hulls, which had previously been camouflage-painted...) some prior camo coat. And chipping the zimm will nicely reveal that under-color! 4) Once on and dry, it is both extremely stable (does not shrink or out-gas over time) and takes any sort of paint well. And last but certes not least... 5) This stuff can be removed by chipping and washing off. VERY useful if you find that you don't like your first attempt(s)...
It's my only way to do zimm.
Just a suggestion!
Bob