Hi
Has anyone used vallejo satin varnish and got this gray overcoat all over instead of semi gloss?
what has happen!
any ideas?

You brush it on or airbrush?

airbush

maybe to thick coat, not the first time

sounds like you have moisture captured in the finish.
If you sprayed on an overly humid day and you put on a thick coat that's probably the case.
But all is not lost (unless you've already tossed the model)
What you need to do takes a little time and patience....
Take straight lacquer thinner and MIST a coat on about every 2 - 3 minutes. Don't put on a wet coat and don't let the finish get too damp with the mists. What you are trying to do is soften the finish enough to release any moisture. It may take a little time depending on how thick your original finish is and how much moisture is in it.
If you work at it slowly and watch it closely you should be able to cure 90-95% of the problem. If you flood the thinner on you will definitely ruin it. Take your time.

Cheers,
C.

As the others have suggested, it's either the varnish not mixed enough, too thick of a coat(s), temperature, or humidity. I have fixed frosted coats by airbrushing a light gloss coat over it. Vallejo makes quality products, so I doubt it's the varnish that "ruined" your tank.
I've gotten all the way to finishing weathering, had something go wrong, and had to strip the tank back down to plastic and start over, so don't feel too bad, it happens to everybody sometime!

What temp and hum, should be optimal for good result?
i was reading that should just always use a gloss varnishes, better for the wash and filters to flow
and yes i will try to put gloss over it again
thanks

I'd say 70º F and 70% humidity would be optimum, but you're good between 60 and 80º F and 50 to 75% humidity. Clear coats are fussy things.