Which comes first?

I rarely paint parts on the sprue, thats just me, but when I do I mask off attachment points (holes in the wheels, ends of axles) and moving parts hinges first. Then primer coat. I'm usually done here, I'll do the base coat when sub-assemblies are complete.

Take one thing at a time off the sprue and add it to the sub assembly. Follow the instruction sheet sequence. Go as far as you can go with assembly until the instructions give you a part that covers up too much of the current sub assembly to reach it all with brushes or airbrushes. Paint the sub assembly. Mask whatever you have to, and continue the cycle. That's the one-thing-at-a-time method which is extremely organized.

Builders also skip through the instructions selecting a larger group of sub assemblies to complete before the painting begins. You have to be more careful & thoughtful about it if you go that route. It can cause you to have too much going on at one time.

i have been painting parts while they are still on the sprue, but i dont really like it as the parts usually ghet dirty again after handling them and putting them togeter. and u have to touch the paint up later, and also its pretty hard to clean a part up while its on th sprue,,, from now on i take parts off the sprue, put togeter what i can, then paint.
its a matter of prefernce tho, just see which mnethod u like best.
ps. small parts which i have no way to hold stay on the sprue tho....

I will spray the interior of parts on the sprue (wheel wells, fuselage interiors etc) as some of them are impossible to paint after assembly. I rarely paint the exterior of a model on the sprue because sure as I do, I'll end up having to do some filling, filing and sanding, which means I've got to repaint.

Painting and assembly order re kind of a matter of personal choice and the individual modeling situation. Things like the interior of a fusulage and related parts can usually be painted conveniently on the sprue. Tank wheels are a lot easier to deal with on the sprue. For other things, I'll paint after assembling into the largest subassembly that is possible to properly paint. You do have to watch when creating large subassemblies that you don't end up with interiro corners that are impossible to spray into.

as others have mentioned here, it depends on what you are building. I plan ahead of time, working in subassemblies and painting everything at once as much as possible, but there are times when that just isn't possible, so you need to mask and/or spray carefully. dryfitting helps as well, as does taping pieces together if you need to do additional work later and need access to certain areas (cabin interiors of trucks for example).