I use Rust-O-Leum Clear Matte, which is the best balance for me, but I’ve heard people have a lot of success with airbrush clear coat media. I’ve used a lot of different clear coat products and found that the method of application matters more than the product itself. Ensure that you apply coats from no closer than about 12″, and do so on a day with moderate humidity (60% is about optimal). Too close and it goes satiny, too far and it goes dusty. Too dry a day and it gives you a weird, frosted look. To damp and it is sticky and will ruin the model. It’s a totally bizarre thing, but you kind of have to roll with it.