Here is a handy tutorial on decal application.
http://www.winterdyne.co.uk/maz/winterdyne_tutorial3.pdf
Covers glossing (avoiding silvering); microsol/microset and best practice when handling decals.
The Lahmian medium is a top coat matt medium. It will not remove 'silvering' which is the most common fault when applying decals. It will hide the 'shine' of the edge of the decal, if it's been applied correctly that far.