Nigel says:

A single action airbrush gives a constant spray pattern when the trigger/button is pressed. The spray pattern/volume is set by a separate knob on the airbrush before you start spraying.

On a double action airbrush, the trigger/button is pressed down to start the airflow and then pulled back to start spraying. The spray pattern is controllable according to how far back you move the button.

You can spend a fortune on an airbrush but I’m using one I bought cheaply on eBay and it’s completely fine for the infrequent use I make of it. It’s a double action brush and it took a few minutes to get used to controlling the airflow and spray using the same button but it soon becomes second nature.

If you’re just looking to use an airbrush to get a superior finish on models, then either single or double action brushes will be fine. For things like weathering where you need to vary the paint flow to get the right effect, then a double action brush is really what you need. I’ve never regretted choosing a double action brush – whilst I only use it for general spraying, I know that it has the extra functionality if I ever need it.

The other thing to remember is that you’re going to need thinners for most paint – many manufacturers make thinners suitable for their paint but just remember that you need acrylic thinners for acrylic paint and enamel thinners for enamel paint. Just aim for the consistency of milk.

Finally, there’s loads of information about airbrush choice and use on YouTube so an hour or two invested will pay dividends.

Good luck!