Yep, just use the back of a sponge in a pinch.

If the call is made out of wood, look at the back and see if you can determine which way the grain is running. You want the grain to run horizontal across the call, and you want to condition the call WITH the grain! Imagine a clock face... go from 9 to 3 or vice versa.

Then when you want to use the call, make sure you run your striker across the grain... meaning on a clock face from 12 to 6. I usually run mine from about 1 to 7 and make a long oval.

Just like a guitar or violin, the direction the grain is in will have an effect on how even your sound will be.... just some food for thought. But it always beneficial to run your striker across the grooves you condition into the call.