I just dug out an old tin of cellulose grey touch up paint, thin on top, very solid at the bottom. I managed to break up the solids into chunks, but no amount of stirring seemed to reduce them, then I remembered my Humbrol stirrer, a bamboo kebab stick, with a small 8mm x 5mm platicard insert at the bottom. Cut the stick down to about 10cm long, chamfer the end to fit in a Dremel and away you go, but even at minimum speed you could end up with it all over the ceiling, so I used a small piece of folded plasticard between thumb and forefinger around the 'chuck' to reduce the speed. It works.