Mr.Hobby - Mr. MASKING CLAY, M-134 
This useful clay-type masking material is a water-based resin clay, so no oil will be deposited on the surface of your models! You'll be able to begin painting immediately, without having to wait for anything to dry. It's easy to work with, too, and can be used for painting camouflage patterns and others; it can also be used for making your own molds. 
-Take the amount you require, knead it well and attach it to the part you want to mask
-Clean the boundary using a spatula, etc.
-If the clay starts to go hard or loses adhesion, add a small amount of water and knead well.
-Paint the part as quickly as possible after masking and remove clay after painting. If the clay is left in place for a long time, it may harden and become difficult to remove or you may not be able to remove clay placed at recesses, etc.