I used a spray called hycote which is pretty cheap and gave a good finish but I don't think they do a clear flat spray so will need to do a test run first like you said with another brand. I always have trouble with the decals, I'll tell you the method I use, I first brush a little humbrol decal fix on the part then soak decal in water and apply then brush decal fix over it and dry with a hairdryer for a few seconds. That's the best results I've had but it's not the best as the decal fix leaves a shine around the decal, I've since seen mr mark softer and setter which I'm thinking of trying?
Thanks again