I have succesfully used Future on both plastic models and on rockets, and if you do it right, it will not yellow. Some people have complained of yellowing in some messages I've read, but usually it traces back to the "white" paint they used for their main coat. A lot of white spray paint looks a bit yellow and dingy. If you can use an airbrush instead of a spray can, try mixing a drop of light blue paint in with the white. It will actually make the white look "whiter"

Greg