I am about to start a 1/48 F117A Nighthawk. I plan on finishing it in flat black. In the past, I seem to recall having a great deal of difficulty getting the decals to affix to flat black and “looking” right. Decal set discolors the paint underneath, so thats a no go.
Any ideas/tips for affixing water decals to flat paint?
Put a little bit of gloss finish where you want to apply the decals.
Do you have an airbrush? When I was heavily into modeling (1/35th armour), I would use a semigloss base coat or whatever colour I wanted, then shott the entire model with flat afterwards. I would also use the flat spray to do some weathering, oil lines, etc. This would help your decals adhere. What kind if decals are you using? You can get aftermarket ones that are nicer, more scale accurate, and more accurate in general. I loved these as they allowed me a little flexibility.
You can also get flat in spray cans, just be careful with overspray and drops.
Another option is to apply the decals to the unpainted model, then cover them with art masking fluid. Masking fluid is sort of like a thin liquid rubber cement that can be painted over the decals, forming a latex stencil over them. You can then spray it with your matte black. Once the spray paint is dry you can gently peel off the masking fluid, revealing the decal underneith.
The problem you’re having is called “silvering”. On a microscopic level, the surface of flat paint isn’t smooth, but covered with tiny pits (these pits are what randomly scatter light, making it look flat). The decals just aren’t flexible enough to snuggle down into these pits.
To get around it, first paint the model with gloss paints, at least the spots where the decals are going to go. Apply the decals, then give the entire model a clear semigloss or flat topcoat.
Lots of good tips. Thanks!
One more trick I failed to mention, you could paint the spots where the decals will go with gloss clearcoat, then use a flat clearcoat over the decal.
In my experience, you should glosscoat the entire model before decalling. I’ve had trouble getting the final finish an even sheen if I glosscoat only the decal area.
Uh, but didn’t he want a flat black surface, like on the F117A?
Yes but then you get into scale effect and that is the sure way to madness. Until I can get the surface irregularities precisely 1/72 scale…
Just an idea I just made up and haven’t tried: Dullcote a glossy and decalled finish, then dry buff it to the reflectivity you want.
I think Rocketeer means 2 coats. Gloss it, put on decals, mask, refinish in flat.
Correct?
Why would you mask the decals - all paint (as seen from a distance) tends to look flat in the real world. Unless you mean the windshields, running lights, etc. (which if at all possible general should be installed last), flat coat the decal too.
A flat or satin clear sure! Thats what I’d probably do. I think it would be a bad idea though if you did a 2nd coat in flat black. Decals may be a bit tough to see then 
One more thing I’d like to add, is that you must mask off any clear parts (canopy, landing lights, etc.) before using a clearcoat. The solvents in most clearcoats will “etch” clear plastic, making it hazy and fogged. (Learned that one the hard way.) It’s best to install these parts last, if possible.