There seems to be a few threads on how to clean numnahs, and I found a way that works for me, so I'll share (my numnah is d.blue, so I'm not sure if this will work for white ones, but I think it would).
Take a brush, and brush away all the excess hair on the bottom of the pad, then wash it with dish soap and a brush, then hose it down (from centre to the outside, so the hair gets washed off the numnah). Repeat if necessary.
Q: I have some fly spray left over from last summer. It's been stored away properly, so would it still be good to use this summer? It's an all natural fly spray, if that makes a difference.
Maci