Pava,white spirit is used for thinning oil based paints,oils,enamels etc... I get mine from B & Q which is a diy type shop that sells decorating materials,it is very cheap by the litre bottle.Cellulose thinner I buy from a car paint shop place,as it is used(or use to be) in the automotive industry etc.. It comes in a large tin but works out very cheap in the long run,I use it all the time with enamels through the airbrush,use it also to clean the airbrush so is exellent to keep on the bench. (very flammable and smells very strong)
Regards thinning Dullcote,it is quite thin out of the bottle but thin it a little more to the consistency of whole milk. I cannot remember exactly just experiment untill it is like milk,it's probally not as much as 50/50 Dullcote to thinner.
Hope you sort it. I never worry about a true matt Finnish now with this stuff !