This stuff might be a practical and readily available alternative to future.
Golden Paints Polymer Varnishes
it's a bit more expensive and requires thinning for airbrushing but is water/ammonia based (like future) and won't yellow in sunlight.
I've also tested airbrushing the matte version and it's dead-flat and doesn't appear to lighten/cloud the underlying colour (like the last time I tried tamiya flat-base with the local supermarket's version of future)
Golden Paints Polymer Varnishes
it's a bit more expensive and requires thinning for airbrushing but is water/ammonia based (like future) and won't yellow in sunlight.
I've also tested airbrushing the matte version and it's dead-flat and doesn't appear to lighten/cloud the underlying colour (like the last time I tried tamiya flat-base with the local supermarket's version of future)