This is Tamiya decal adhesive. It is perfect for giving adhesive power to decals which have sat too long in water and lost their stickiness or become prone to "silvering," when the clear face of the decals whitens.

Simply apply to the area of the model where the decal is to be placed, and then apply decal.
Comes in a 10ml bottle with cap brush for handy application.
Water-based liquid makes for safe work, and can also be removed with a wet cloth if it escapes from under the decal.