I have been able to remedy this with subsequent coats of ulthuan grey and white but it means a redo of the casings essentially after the model is finished, a little frustrating to say the least.
I had a think about this and tried sealing the figures with varnish before painting the bolter casings, I tried his with both matt and gloss varnish with no success. It looks like the long way round is the only way, I will continue to post as the bolter casings are redone, for now here are a few more marines, I intend to have two squads of ten rotagun marines as the Thousand Sons seem to make best use of them and they look cool.