Howdy, Slinky!
That question pops up every now and then, and my answer is always: if the tanks is going to be built "clean" you could paint the tracks separately. Or if the track is of the "movable link type", so you can install it after painting without glueing. But if you need to glue the tracks on the model to get the correct shape, or the tank is to be built "dirty", I'd recommend glueing first, painting later.
Now I can't tell you things like "take a brush this fine and insert it at precisely this angle", can I? But I have some WIP photos to show you what I mean.
Let's take my 1:72 M48A3 as an example. First you assemble everything - I use lacquer thinner for glue, works best/at all on unpainted parts:
Then I went on with two color preshading (using an airbrush) - the base was Humbrol 113 red-brown, panels and road wheels were picked up with Humbrol 66 (my color for US OD):
Once this dried well i went on to simulate some wear effects. From what I've seen on the photographs, uder certain conditions the
surfaces of the running gear being rubbed constantly maintain the
colour of the material (in this case rubber or steel), the other
surfaces remain dusty red-brown. That's what I want to get, so I took a
fine brush and started applying Humbrol 32 (Dark Grey - my favourite for
weathered black) and Humbrol 53 Gun-Metal. When I was done, this is
what it looked like:
Add some dry-brushing and maybe a wash, and it will be OK. Maybe you feel like what you said before - OK, I still don't know how to do it - I say give it a try, you'll be surprised. In most cases, what you can't paint won't really be visible, anyhow, so what's the big deal? Just take a fine brush and try to relax and keep steady, and it should do. Hope it helps, good luck with your project and have a nice day
Paweł