AMMO by MIG Accessories - Masking for Modelling
Item: Ultra Masking Liquid

From the Manufacturer:

Liquid mask for covering and protecting surfaces and features when painting. This product is ideal for the utility of masking clear parts or creating multi-colour camouflage patterns. You can apply it with a wooden stick or a sponge to simulate chipping and wear. Use it straight from the jar and allow it to dry approximately 20 min. To remove, gently rub the surface until the masking is removed. This product will not attack paint or damage delicate pieces. Blue colour.