Varnishing my spitfire

I admit I havent used Humbrol but being water based shouldn't effect the enamel paint. It's always best to test paint combinations on a piece of cardboard , plastic or a "paint mule" ( a discarded model you're never going to finish ) before using it on a finished model you've worked so hard on. It's well worth the time and effort to do so and it's faster in the long run than striping and repainting your model when after the paint combo doesn't work out.

Having said that, there's two products I use interchangeably, Testor's Dullcoat Laquer ( thinned 50% with hardware store laquer thinner) and Alclad's Aqua gloss. Airbrushed  with very thin coats ,these products have worked well for me not having any problems with any type of paint underneath.  I hope this helps Mate.

I haven't used Humbrol either, but you shouldn't have any issues with putting it on the enamel.

Is your Humbrol Flat coat/Matt Coat labeled acrylic? I have been using their enamel Matt/Flat coat for many years as my go to product. The stuff is excellent in my experience. I thin it with Testors Universal Enamel Airbursh Thinner for best results.

Thanks for the suggestions.

stikpusher

Is your Humbrol Flat coat/Matt Coat labeled acrylic?

It says "water soluble", which I took to mean the same thing (but perhaps I'm wrong about that).

Good question... my next question is- is the stuff that you have in a tin, plastic pot, or glass bottle? I have the enamel stuff in tins and larger glass bottle. 

It's a 125ml glass bottle.

I've been using both the flat and the gloss for a while now , had no issue's with either ,  I have thinned it with both lacqueur and tamiya thinner's , I have also used it straight from the bottle .