Don't put celly onto enamel or any other oil based paint it will wrinkle it . You can use any varnish to seal. Humbrol Marine varnish is also hot fuel proof

Two pack epoxy is the toughest!!

it largely depends upon many factors what are you going to be doing with it. For a scale gently handled model acrylic is fine for something to be handled roughly then enamel ( or cellulose ( if you can find it)). Just remember that the surface your painting on counts cellulose onto enamel will "pickle" acrylic you can paint onto just about any finish. Seal it all with an acrylic varnish.