Many thanks for the replies so far. I think ErikB refers into one Machinen Krieger series of Japanese mecha models and associated model figures of ~ 1:20 scale. Wmyers thanks for the detailed info, I'll be checking out the water based paints and Mr Surfacers in some not too distant future.
I use P3 Formula acrylic paints by Privateer Press (think: Citadel paints of the 1980s-1990s) and they're excellent for 28mm figure painting, but can give a bit blotchy results with some of the white or very light colours. Like, you dip the brush tip into the paint pot and it's half dryed up already when you move the brush tip from pot to the model figure, trying to paint eyes and teeth (like the orc face on my avatar picture). As I have a WH40K Eldar collection in need of paint, with plenty of models that have large, smooth surfaces, I'll check out if the Sangyo acrylics would be of help there.