That was my question.
My usual order of fleece cleaning activity is
1) Skirt heavily!
2) Soak is Fermented Suint vat in the back yard
3) Rinse fleece (in rain or bucket with rain water)
4) Lay out to dry in the sun
5) Comb locks
6) Scour in mesh bags (hot water and Unicorn Power Scour or Dawn detergent)
7) Rinse (hot water)
[9) Make rolags with hand cards - optional. But typically I do need some other prep step after scouring and before spinning.]
Why comb first? It gets a lot of the dirt out and also gets out any short bits or second cuts hiding in the locks. Recall that I am literally combing, with a small dog comb (see photo). Having removed a lot of excess - and having determined if the fleece is weak/broken and not worth washing - I save on water and product for cleaning and my water stays cleaner.
But I wondered: would I save myself time if I combed after scouring? Then I wouldn't have to add a step 9 to prep the fibre for spinning.
I made a comparison
Along the bottom is a progression of combed locks (and the leftovers beneath), scoured locks, and carded locks. It's soft (and I'm spinning up about 30 rolags right now.)
Along the top is the experiment of scoured (uncombed) locks. You can see the scoured lock is still quite dirty. I don't expect washing to get all the dried dirt out - or, if I did, I'd waste a lot of water and soap trying. Why do that when a quick flick with the comb gets the dirt loosened out? I wondered if the dirt would release more easily. I don't think it made much difference. Sure, the waste part looks less dirty than the waste below, but who cares about the waste? Ultimately, the scoured-then-combed locks don't look any cleaner than the unscoured, combed locks. In fact, I combed all the scoured locks and tossed them back in another wash I did today. They just weren't clean enough for me.
Combing open the tips and releasing 90% of the dried dirt goes a long way in cleaning the locks, so I will keep this method. Between the FSM vat and a quick combing I've never needed more than one scour to get the grease out and whatever dirt remains. (Or I spin in the grease.)
However, a current bag of locks drying will test the one-scour method. I noted while skirting that this is a particularly greasy fleece. (The first fleece of this season was not greasy, well, very lightly.) It was so greasy that I couldn't really do a thorough job of combing. I opted instead to make sure all the tips were open and most of the dirt had fallen out before tossing in a mesh bag. Then I put it through some power scour (along with the remainder from the above experiment, in its own bag)... I am eager for it to dry.... is it still really greasy?? somewhat greasy? not greasy??
Dry fleece! Dry!
(patience, I'm still working on it.)