Airbrush masking putty from greenstuffworld is my new favorite thing. Reusable silicon putty which leaves no residue and isn't reactive at all (unlike blutack and some masking tapes). If you like playing
Big mistake using AK Interactive Camouflage putty for masking - Painting & Weathering - KitMaker Network
Silly Putty masking | The Butterfingered Modelbuilder's Adventures
How to paint hard edge camouflage with masking putty – HeresyBrush