I think Humbrol makes a clear varnish that dries to satin finish. Anybody else? Humbrol paints are harder to get. A local dealer has a very depleted shelf with most of the colors missing. I’d have to order online, which is something I hate doing with paints because I like to hold them and examine them before purchase.
I really like JW’s right step. It is a water-based clear acrylic varnish, usually I get this at my local Michale’s art supply in the acrylic paint section. It comes in matte, satin, or gloss. I airbrush it mixed 50/50 with distilled water, and it dries dead flat (the matte does anyway). Just make sure to stir it with a wooden stick or something, as it does separate, and shaking it causes bubbles, which take a LOOONNGGG time to settle out!
Chris