you could always shoot another coat of clear on it with a higher ratio of gloss to flat your 50% mixture had too much flatting agent obviously, you might start with a 10% flat to gloss ratio, and just do a brush out on a smooth surface adding small amounts of the flat clear untill you have the shine level your looking for, many paint stores will give out brush out cards for just this porpose there handy to have around if your doing some painting.

unfortunately flat sheen is achieved by having a surface texture, it may seem smooth but there is a micro texture there for the dull finish to difuse the light, no matter how much rubbing you do your going to have a hard time getting s/gloss from it without some major elbow grease, even then it will be tough.

it shouldn't take a thick coat to gloss it up just a light spray over the entire surface should do the trick once you have mixed the materials to your prefered sheen, gloss level is achieved by the ratio of "medium" to flatting agents, the medium is the resins that make up the paint for the most part.

when I am doing any model paint work I use a graduated syringe to draw the paint from the can and thin for spraying, when I am using water based materials, which is most of the time now so I can get accurate ratios for spraying.

when I am spraying models I use a higher end airbrush which takes a little less material to get it working, and it has a little more product control,but those guns work great as well its a learning curve.

I use the same principal for when I am shooting paint for work,BTW the plane looks great