When I had already applied some of the decals for the Nova class, I noticed that the decals for the sensor arrangement behind the bridge do not fit into the recess. So I filled it up.
First I always apply a thin layer of matt varnish and then I put the wet decal on it. Then I slide the decal into its final position and pat everything dry. After drying, I apply a thin layer of matt varnish over the decals. This seals everything and protects the decals from UV radiation (fading and yellowing).