I had a similar problem with the decals that came with my Monogram P-39. Here's what I did:
Soak the decal until the glue lifts from the back of the film (after the decal has come free from the paper.
Using a wide paint brush, gently brush the excess goo off the back. Keep going until it looks like it's all gone.
Apply the decal to the surface, using setting solutions as needed, or
Use Future as a decal solution. Remove decal from the water bath, touch to paper towel to remove water, then dip in Future and apply to surface.
Seal with Future.