There are multiple ways to sand the playing surface.

One of the best ways is to actually draw a line down the middle of the call. On one half, sand north-south, and then on the other half sand east-west. When you move the striker in a circular motion, you will actually change pitch/tone as you move from a perpendicular to a parallel direction of grain.

Also, do not be afraid to circle those sweet spots on your call. I typically use a fine point sharpie (in red or blue), but only on commercial calls, not customs. I don't really find any particular "sweet spot" on a custom call, it plays well anywhere, for the most part.

Sand the strikers hard and often, period. Nothing sounds worse, IMO, than a glossy striker squeaking across a friction surface...ugh!!

I just maintenanced my calls tonight with my daughter, and will maintenance them again as needed. If the weather is humid/wet, or I use the friction calls often, I may rough the calls & strikers every other day during the season.

Cheers,
S&R