Today I've begun to test the camo masking sheet proposed by Eduard. 4 parts are used for the undersurfaces; see photos 1 & 2.
Useful, but for the undersurfaces I would have preferred more masking tape as you can see on photo 3.
But that masking sheet will be useful for the upper surfaces!

"Bubble" is used to test that masking sheet; I have airbrushed the Ocean Grey.
Now, to mask the undersurfaces of "Malcolm"...