I painted my first plastic soldiers at age eleven or so, using Airfix paints (good riddance...) and a TOOTHPICK... (no pics available, sorry). I did not prime them. Since Airfix Waterloo figs came in a light yellow plastic I thought that was fair enough to represent skin colour...
Then I used clear gloss water soluble varnish to make the colour more durable because after all I wanted to play with the figs. I was extremely annoyed that the colour kept coming off the figs... Thinking about it that was the main reason why I gave up dealing with plastic figs for a very long time. I was not pleased with the lack of historical accuracy either (yes, that was indeed important for me when I was 13
).