Thanks Gents!
It's a water based wash I'm doing for panel lines, which will be over Future or Li quitex acrylic clear (In turn over Tamiya acrylics).
I've been messing around with Liquitex acrylic clears as Klear (Future) isn't as easy to get hold of here & also because I've some concern over yellowing with Future, especially over a white base. Liquitex advise that their acrylic varnishes shouldn't be thinned, even for airbrushing, but straight Liquitex "High Gloss Varnish" doesn't level out anywhere as well as Future, I've tried adding some Flow Improver which gave some improvement, but not enough.
I've read a few snippets (mainly from artists) that Liquitex clears thinned with Tamiya X-20A works very well, so I'll be giving it a try.