Help!

High humidity can cause the " Frosting" .

How long did you wait between coat's ? It may not be compleatly dry yet. sometime's the "frosting" dissapate's if you wait a day or so.

When this happen's to me, I rub down the "offending"area's with 90% isopropyl Alcohal.

TEST FIRST !!! The Alcohal can and will take off the Gloss-coat too. Be careful.....

Test ...   Test  ...  Test .....

When I’ve had the frost happen in my flat coats, it’s usually because I did not thin it enough. I’m usually working  at a ratio of 1 part flat clear to 2 or 3 parts thinner at a minimum. Sometimes more thinner to flat coat. It all depends on how much I want to tone down the gloss. 

Roald

m using Tamiya clear gloss and clear flat thinned 50/50 with Tamiya lacquer thinner.

Are you using Tamiya XF-21 Clear Flat BASE or XF-86 Clear Matt coat?  They are 2 different products.  If you are mixing your clear gloss with XF-21 50/50 the reason it is turning white is because you are using too much XF-21.  It will "frost" or turn your clear white if you add too much.  You only need to add about 15-20% to gloss to get it to turn flat.

Bronto

Are you using Tamiya XF-21 Clear Flat BASE or XF-86 Clear Matt coat?  They are 2 different products.  If you are mixing your clear gloss with XF-21 50/50 the reason it is turning white is because you are using too much XF-21.  It will "frost" or turn your clear white if you add too much.  You only need to add about 15-20% to gloss to get it to turn flat.

I'm using X-22 Clear as the gloss coat,  then decals, then XF-86 Flat Clear mixed 50/50 with Tamiya lacquer thinner as the dull coat.

Given that you're using XF 86 flat clear rather than the troublesome X 21 flat bae, aanotjer possibilith is that you may be airbrushing at too great a distance from the surface or at too high a pressure, allowing the clearcoat to partially dry befoe it eaches the surface. This may be exacerbated by high ambient temperatures and/or low himidity.

Your immediate problem of frosting may be relieved by simply applying another  light coat of clear gloss.