advice needed for AB newbie

thanks for avdvice .Have stripped model down for a third time and have made adjustments of thinning paint more and backed air pressure right down to less tan 5psi according to my regulator .This time all is well.Understanding the cause was key in working out this problem.Will try light coat of thinners if it happens again. THANKS AGAIN TO ALL

The problem indeed is a too quick drying of the paint. Actually you're throwing pieces of paint on the model that stick, but do not have the opportunity to flow into eachother, making a neat coat. This results in the orange peel.

One way to sort this out is too make your paint some thinner, or the suggested pressure-tinkering.
An other, which I used to make a smoke-coat on a S-3 Viking canopy, is to first spray the orange pee paint, but directly afterwards spray the subject with a coat of pure thinner (which you have to anyway to clean you airbrush). The orange peel coat probably hasn't hardned out yet, so the thinner will then make a fluent coat. Be sure not to use too much thinner, as it will make the paint drip and run over your model. So be delicate!

Take a peek on my website, under built models, S-3 Viking for the smoked canopy.

yes have been using tamiya thinners thought i might be over painting the model as undercoat (black) looked smooth when finished but when basecoat is added i end up with the dusty/griny loook

What are you thinning the paint with? If you use Tamiya thinner, there is a retarder in the thinner to slow down drying time.

Back you pressure off to 15 or 18 psi and add a drop of liquitex flow aid to the paint. Your paint is drying too fast - the flow aid will slow this down and eliminate the problem.