[FIGHTS-ON]

Always used Humbrol matt varnish now. Other varnishs i have used have occasionaly left the same 'white' stains. One way to beat it is to really thin the varnish down, and be prepared to 'mist' on the coats - maybe as many as 3 or 4 times (i.e. after 1st coat you'll only notice a slight dulling of the finish.)

I expect you've already tried this approach.....