I'm currently working on Dragons Zimmed Tiger I Ausf E - the Zim effect on that is quite pronounced and it took a fair bit of effort to get the decals to sit.
Once basecoated and cammo'd I just used the normal method of adding spots of Gloss varnish through the airbrush on all the areas where decals would be going.
Once dry the decals went on and Kitchen paper was used to soak up excess water then after a few mnutes of dry time plenty of Micro Sol was added by brushing it gently over the decal - and then a bit more too! Be careful when brushing it on as you don't want to damage or move the decals. I then seal over the top with more varnish when the decals dry.
I can imagine a putty Zim job will be tricky so I'd definately try Gloss varnishing the decal areas first - or stencil painting like you say.
Ben