New to Airbrushing and having a bit of a problem.

Gloss acrylics require a little more thinner than usual. A good test is to spray a card board box and see how well the paint is flowing. Start out and see if 50/50 is good enough. Trust me you can tell when the spray is actually smooth. To get a consistent coats without drip just go nice and even strokes. Unfortunately airbrushing is an art in a lot of ways and requires some practice and experience. It will probably cost you a few bucks in paint to get the stroke down. Another great way you can test and practice airbrushing is going out and buying a cheap model kit like AMC model car kits you can get at Hobby lobby.. Just bring a 40% coupon with you and you can get a practice model kit for less than $10. You can sand and repaint that model all you want. I used this technique to practice gloss coating.

Thank you, will do just that. Lol, I do use X-20A. I'm using the Harder and Steenbeck Evolution 2 in 1.

I have practised on paper and the scraps of plastic the models come attached to, The only problem I'm having with it is the larger parts of the model (the shell etc) .

Guy.

Tamiya gloss acrylics can be a little problematic to airbrush. It sounds like you're getting some roughness from the overspray at the edge of your spray pattern. Airbrushing gloss is a little tricky - not enough coverage and it's rough and patchy, too much and it runs - only experience can tell you how much is enough.  

As suggested, reduce your air pressure to 15psi, thin the paint some more (maybe a lot more) and get up close and personal. Do not be tempted to use alcohol or another substitute thinner as these tend to reduce the gloss of the finish. Use Tamiya's proprietary X-20A thinner or, when you get a little more experience, Tamiya lacquer thinner.

Drop the pressure (about 15psi if you're using the Evo) & increase your percentage of thinners.

Orange peel is an indication that the paint couldn't flow together & create a smooth surface - increasing the % of thinners will help this & a slightly heavier coat may also help if the thinners alone wont solve the problem. You would save yourself quite a bit of heartache by messing about on some scrap until you can get the balance between orange peel / runs just right.