I have Matt cleared the turret on my T90A on the top one half is Matt wI'll other has gone semi gloss .I would usually put paint or varnish to it looks wet so I'm not sure if temp (been spraying and 15-16 degrees )or putting to much on and I'm spraying at a pressure of about 20 PSI
I'd be wary of thinning any vellejo paint with an alcohol based thinner like X-20A. It sometimes causes the paint to congeal. However I've never tried it with the matt varnish because I use Gunze Matt.
I've always used the tamiya clears and never had an issue. I do have a battery powered paint stirrer which might help but otherwise I mix approx half and half with x20a thinner and spray away. A trick to get a better finish is to spray a mist coat of straight thinner right after the clear coat, this helps it level out nicely and get a smoother finish. But lately I've been using the gunze aqueous ones and they perform in much the same way.