I have a long haired German shepherd who produced such lovely soft undercoat when groomed that I thought I'd have a go. I didn't card it, but spun it straight and unwashed. I got some lovely soft fibre, but it was not a lot of fibre from several carrier bags of fluff so I've focussed on my sheep's wool which I have by the sackload! Some dog hairs just don't have enough crimp to catch together either so would struggle to spin. I'd say just have a play and see what happens!