I tried the water-based clear (Minwax Polycrylic) as a clearcoat but it softened where raw gas got spilled on it. The oil-based urethanes are more gas-proof but will yellow your colors. What I am doing on my present plane is spraying the majority of it with the water-based stuff and just doing the bottoms of the cowls and nacelles with the oil-based stuff. I think the yellowish, dirty look will look scale around the engine areas.

Jim