Help please! Flat finishing paint problem.

Please could anyone help? Recently, I finished my Tamiya Spitfire Mk1. After applying a coat of gloss on the painted surface, I applied the decals(with Micro Sol and Micro Set). Then I airbrush a coat of flat clear(Gunse H20) to seal the decals and to make the surface matt. Problem then occurred, some white matter deposited in some of the surface when dry. I was sure the paint was new, the airbrush had been cleaned through, so what caused this.

I did the similar steps in my Challenger tank without problem(perhaps it has but due to the light tan color on the tank I couldn't see it)

Now I suspect the following problem(s):

1) Paint is no good.

2) Paint is inappropiate. Perhaps H20 is no the best. BTW haw anyone used Microscale flat finish? Any comment?

3) I mixed the paint in a wrong proportion. I mixed them in 1:1 ration. Perhaps I should dilute the paint more.

Can anyone give me some suggestions. Also, would it be wise to add an extra gloss clear on top of the decal first, then apply the flat finish? Thanks for you help and attention.