From Bob I: As many know I'm big on sanding, filling, resanding and repeat again and again. I use finishing bondo in the tube that I get from Canadian Tire in the Automotive paint section. I like it as it's very fine for small holes. I use plastic oil painting knives from Michaels. They come in a package of a few different sizes & blades and are not expensive at all to apply the bondo. Applying thin layers are better than thick as it dries very fast and can be faster with a hair dryer. Also, when finished sanding it takes paint very well.

The most important trick I've learned about sanding is to wipe the surface frequently with a soft micro cloth, then I'll close my eyes when I'm feeling the surface for deficiencies as your eyes will fool you but your touch won't. 

Disclaimer: I have been learning the art of airbrushing for the past 6 or 7 years. There are many differing opinions out there and dozens of really good (and some bad) YouTube videos. My presentation may not illustrate the best way to use an airbrush but it describes the best methods I have found to work for me by trial and error.
Why use an airbrush? – I decided to use try airbrushing when I was building my WWII Corvette Convoy Escort as it had some great detail including photo-etch parts. An airbrush will lay down a good covering of paint in a very fine layer so that all detail is retained.

Kinds of airbrushes
Gravity feed - In a gravity feed system, paint is drawn into an airbrush from a cup mounted on top of the airbrush. Gravity allows the airbrush to spray at lower air pressures. This feed style requires smaller amounts of paint and utilizes every drop, making it economical to use and easy to clean. Most gravity cups include a secure lid, which prevents paint spills.
Siphon feed - In siphon feed systems paint is drawn up from a jar from underneath the airbrush. Unlike gravity feed, the air compressor must do all the work. This means more air pressure is required to spray material. Siphon feed is preferred when using larger amounts of spray medium.
Single action - Single Action airbrushes have independent controls for air and paint. The trigger only controls air. A dial on the handle sets how much paint is released when the trigger is pressed.
Single action airbrushes can be good choice for a production environment when a spray pattern needs to be repeated exactly time and time again. Sometimes single action is recommended for those just learning but new users quickly outgrow single action and usually desire more control.
Dual action - Dual action airbrushes allow you to control both air and paint flow at the same time. Pressing down on the trigger releases air and drawing back on the trigger releases paint.
Pull back the trigger slightly for a little bit of paint and pull back farther to release more paint. Dual action airbrushes give you much better control and can create more dynamic spray patterns.
External vs. internal mix - With an external-mix airbrush, the paint supply is pulled by siphon from a reservoir into a stream of air and dispersed via air pressure. These designs are extremely rugged and well suited for broad application. Fine detail work will be very difficult. Careful thinning is required to avoid a spatter pattern.
With an internal mix airbrush, paint is drawn via siphon or gravity feed into a chamber (nozzle) within the airbrush where it is mixed with air before exiting. Control is achieved by varying the paint flow. These designs produce a finer paint spray, but most are designed for relatively fine work (not spray-gun).

Paint
Acrylic - Most paints sold for models are acrylic as cleanup is easy with a mild cleaner and water. Odour is almost non-existent so airbrushing indoors is quite acceptable. Acrylics dry in 10-15 minutes or faster with the help of a hair dryer. I use Tamiya and Vallejo paints. Tamiya has more gloss colours for model cars whereby Vallejo has excellent military flat colours for ships and tanks and the like.
Enamel – While much more durable, enamels are smelly, take longer to dry and are challenging to clean up.
Synthetic lacquer – Lacquer paints again are very durable but the thinner and the paint is very smelly and rather toxic so not for use in your workshop unless you have a very good exhaust system. The thinner, however, is a good cleaner for the airbrush
Craft – Craft paints are cheap and plentiful, even available at the dollar store. I used them initially in the interest of economy and found that they spattered and clogged the airbrush. I am told that the pigment size is too large for the small orifices in airbrushes. Many people swear by crafts paint, I swear at it!

Additives, thinners and cleaners – Do I really need them? Yes…
Flow improver – I was suspicious of this product as it just sounded like a way to get me to spend more money but I did plenty of research and flow improver DOES improve the flow of paint through the nozzle by slowing down (slightly) the drying time of the paint. 4 or 5 drops of this product eliminates a buildup of dried paint at the nozzle.
Thinner – I have found that all commercial paints need to be thinned, more so if you wish to paint at lower pressures. Thinner is less expensive than paint so it makes the paint go further. As a rule of thumb I use 2 parts paint to 1 part thinner for most paints and half & half for Tamiya clear coat
Airbrush cleaner – Water will do for cleaning but a proper airbrush cleaner will dissolve the paint well and flush it out of the airbrush. Very little is needed and final cleaning can be done with water.

Equipment
Airbrush – There are many brands out there and, on the internet, there are really cheap ones. Some people have success with the cheaper ones – I did not. I now have the Iwata HPCS which is a gravity feed, internal mix, dual action airbrush and I am very happy with it. It is the third or fourth I have had and I would buy it again.
Airbrush stand – I saw this used on a few videos and again thought it seemed frivolous but then decided to try it. Very useful and helpful when cleaning the brush.
Compressor – Any small compressor will do as long as you can tune the pressure down to 20 psi or so. I used my pancake industrial compressor for a while and it worked fine but the noise was horrendous for working inside the house so that, when a real airbrush compressor came available used, I jumped at it.
Turntable – A simple home-made tool I find useful for almost all spray painting
Parts holders – I use small alligator clips on the ends of cocktail toothpicks for most painting. Also, I use tape with the toothpicks.

It's possible to spend hours and hours watching Youtube videos about airbrushing. Nothing compares with seeing and learning in person, and at the November meeting Paul got "close up and personal" ( as close as Covid restrictions allow) about airbrushing and passed on all he'd learnt of the techniques.

Sanding ad infinitum

Paul's techniques in airbrushing.

Building Tips - Painting and finishing