[h=2]Try this old fatman
Surface preparation[/h]It is vitally important to have a gloss or semi-gloss surface to apply the decals. If the model is painted with matt/flat paints, then the surface needs to have a couple of thin coats of gloss varnish, or Klear/Future to give it some shine.
Matt/flat paints leave a surface that is microscopically rough. If a decal is applied to such a surface then it will only make contact with the high points on the surface of the paint. When dry there will be thousands of microscopic air pockets under the decal which will appear as ‘silvering’ and will look terrible.
Some modellers will only varnish those parts of the model where decals are to be placed, but there is a danger of producing a ridge at the edge of the varnish. Most modellers will varnish the entire model because this is not only needed for decal application, but also makes it easier to apply filters and washes to the model.
The model can always be returned to a matt or semi-gloss finish after decal application with another coat of appropriate varnish.