I normally manage OK with ordinary white spirit, but...... Sometimes there seems to be a breakdown in the paint makeup itself and one cannot get anything better than a thin sludgy paste.

Paints themselves and new format thinners & paint strippers all have altered due to regulatory 'improvements' of ingredients, and this does seem to make them incompatible with older paints.

Incompatible in what way?

I am gradually using up a large stock of old Humbrol tinlets I inherited from my Dad (some of which are well over 40 years old and annoyingly includes some quite useful colours that don't seem to be available anymore) and haven't noticed any problems using modern thinners? I mostly use Xtracolor XDTT quick drying thinners from H.G.Hannants of Lowestoft (a hangover from my military modelling days).

http://www.hannants.co.uk/product/XDTT

Thanks Adams 442 and TT Pete.

White Spirit will thin paint, but it does it by breaking it down and effectively destroying it. So next time you want to use it it will probably not mix up properly.

I will try the Xtracolour as an alternative.

All the best