I have an answer for you "Corn Starch"
Work it in like it is powder you need to get to the skin. It makes working undercoat out much much easier. It makes it easier to split the matts (you need a matt splitter for this and use it carefully.)
1. apply cornstarch
2. use matt splitter
3. use under coat rake
4. top off with slicker brush
(This info courtesy of the Late Barker Sisters - both long coats.)