Rod wrote:
stuart noble wrote:
snip
Probably microspheres/glass bubbles. 3M makes them I think.
Perlite would be cheaper


So just mix in a bit of lightweight filler. :-)

(Acutally, how would anyone know if the microspheres contained air or
not? Not exactly easy to measure for ordinary mortals.)

Only that a one kilo bag of them is huge.
Stirring them into paint isn't as easy as it sounds either :-)