When airbrushing a base coat, I do my best to cover the entire surface of a model. However, at a later time I almost always find underpainted or unpainted spots. Is this a matter of experience? Or are there some tips to help a person like me?

Three things:

1. Bright lights
The brighter the better and from.different angles if you can get them.
2. A pattern
Paint each model starting in the same place and then follow a set pattern, it will help.to look at the tanks.you have done and pay special attention to those areas.
3. PRACTICE
What can I say but the more the better. Always hels me and if you do miss a spot or two mud, dust and filters are your friend

I always apply multiple coats. Three is the most I have done. It helps to make a smooth, rich base with little chance of having missed a spot.

Use a brown undercoat, as it covers the gret/green/beige plastic better, and then if you miss any of this, it looks like dirt.

Yes. Multiple coats and a contrasting undercoat--contrasting with the plastic, I mean. It's loads of fun painting one of Tamy's panzers molded in tan plastic in any shade of German Yellow! Gotta paint that sucker black or dark brown (I use hull red as a pirmer for most of my ww2 german stuff, as an example). The advantage of doing this is 1) you get a degree of preshading depending on how skillfully and subtly you apply the top coats and 2) if you miss something, well, the germans often painted their own vehicles in the field and I'm sure they missed something from time to time!!

I will usually paint the hard to reach places first, and then go over the whole thing. That goes for primer and paint. Jeff T.

Think of it as an art piece. Everything has angles , do all the edges first, fill in the flat surfaces last. By the time you do all the angles, the flats won't need alot of paint. Be patient.
Jam your airbrush into the wheel compartment & blast it, use a brush to move the excess around. Or you can paint the wheels separately.
I usually put 2 light coats of black primer.Inspect your paint job after the 1st coat , you will see your light spots. Focus on your light spots during the 2nd coat.

I do the same as well, nooks and crannies and the like. Then i paint about 3 med./thin coats with Tamiya.

I agree, cut in hard spots with brush first. Or prepaint the parts , assemble , paint again.