StanG

I'm having real trouble using Xtracolor paint, Gloss Sea Blue (I'm using brushes btw).
Firstly, I thinned it a little with Humbrol thinner which I could not stir so I bought a new tinlet.
I have not thinned this tinlet at all. I'm painting it on blue plastic Revell kit (Corsair). It's not bad, but it's not looking great either.
My next model is a Corsair again but by Tamiya. This is grey plastic. I have tested it on the sprue and it just makes it glossy gray. I will have to undercoat it with some blue paint and then put at least two coats of the Xtracolor paint.
I'm am testing other paints prior to painting the Tamiya kit on its grey sprue.
1. Humbrol Midnight Blue (gloss), first coat and the grey is gone and the paint looks very nice.
2. Humbrol Navy Blue (matt), first coat and again no sign of grey plastic any more.

Am I doing something wrong here? Anyone experienced this? Could it be a bad batch?