I haven't been writing much lately because I've been busy. I have ripped through a lot of minis, lately, painting them in batches. I'm taking advantage of clear weather, good for painting, priming, dipping, and sealing.
For me, this was the most important basic painting lesson:
Paint reveals details, it does not conceal them. If you put some paint on a mini, the little details pop out. They don't get covered by the paint but rather show in better relief. In fact, you can often figure what some obscure and tiny detail is by sticking a little paint on it and see what shows through.
Corollary: Mold Lines Must Be Destroyed. What I learned from the lesson was that you can't paint over mold lines and conceal them. In fact, in my experience it's better to over-file and over-smooth a mini and wipe out mold lines with extreme prejudice than to leave them. You'll see them later, for sure, and that irritates me. Sometimes I'll prime and then re-file and then spot-prime before I start painting.
There is probably some flimsy way to tie this to gaming in general - "details reveal, they don't conceal" or something like that. But I'm too tired to think of one.