| It is hard to know exactly what the problem is. The paint to which you applied the decals should have gloss finish and be well cured. (You may have done this). The decals may just have been a poor quality, or simply not needed a a decal setting agent. Decals do wrinkle a little under a setting agent. If you leave them they should settle down to tthe contour of the surface when dry.
Humbrol matt coat has to be really well mixed; you get a sludgy goo at the bottom. I only apply it by brush - I would't dare put it through my airbrush.
Having re-read your post, are you matt-coating before decaling? If so, why? The paint job should be glossy (either a gloss paint, or futured), then decals, then seal with another gloss coat, then weathering and panel lines, matt coat, more weathering if required. (that's my system). |