Good stuff Igor. The 'whitish stuff' on the clear film of the decals is the adhesive, which has 'gone off'. This can be cured by slipping the decal off the backing paper, turning it over on a finger or thumb, then carefully brushing off the milky adhesive, using a wet brush. Then, place the decal as normal, but use a decal solution and/or water on the surface. Press in place, then add a little more decal solution. They should then be fine when sealed with a clear coat.
To see this in practice, have a look at my build of the 'Bob Cross Spitfire MkVIII', where I had to do this on 30 year old 'Almark' SEAC decals.
It's a pain, but works !!