Putting decals over zimmerit can be tough. This following procedure would probably achieve good results but there's always the chance for air to get under the decal.
1. use future to prepare the area where the decal is to go
2. use a decal - set solution then apply the decal
3. use a decal sol solution and allow it to dry over night
4. repeat step 3 one or two more times until the decal finally lays in the ruts
5. dull-cote to seal the decal in
Many people will not even attempt to decal over zimmerit but will paint their crosses and numbers on instead.
Happy Modelling and God Bless
Robert