I think sebhorrhoeic dematitis is just a skin manifestation of the spondylarthropathies. I've had it on and off for years. Flaky scalp, but only in reddened patches - not all over. If it's raised and nasty enough some might reclassify as psoriatic arthritis.
If I can keep my diet as strict as I can it becomes less obvious and patches on my face become only barely visible. The distribution on my face is usually along the eyebrows, on the upper lip, and just to the side of my nose.
Early on in my disease process I had a number of boils (and two carbuncles) on my scalp - usually secondary to scratching the dermatitis.
Hope you can get on top of it soon.
Good luck - David
Dx Oct 2006 B27+ undifferentiated spondlyarthropathy (uSpA) with mild sebhorrhoeic dermatitis and mild Inflammatory Bowel Disease (IBD) controlled by NSD since 2007.