A couple of things.................

Water based polyurethane is FUEL RESISTANT, not fuel proof - it is compatible over every type of paint I have encountered . After the residue sits on the finish for about 3 hours the finish will soften. This MAY be acceptable to you if you clean your planes within the 3 hour window.

Oil Based polyurethane is fuel proof (after about 2 -3 weeks), however, it yellows. The yellowing starts being noticeable at about 6 - 8 months. It is not that noticeable on dark colors, but very noticeable on light colors.

Nelson paints is no longer in existance, however Warbird Colors, I believe, has a clear coat that is the same thing as Nelson's was.

I have also used "High Temp Engine Enamel" with very good results. I have used this over a mix of acrylic, regular enamel and latex (I THINK it is an acrylic). I LIGHTLY MIST the first 2 coats and let it completely dry (overnight), then apply the final clear coat. This can normally be obtained in a rattle can at an auto parts store - I have only found it in high gloss though.

The suggestion of spraying a clear coat of acrylic clear is a good suggestion also.

In any case, do a test panel before commiting to the plane.