Might lightly spray the wood wth water then lightly sand off the raised grain... before treating. After treating sanding is more difficult.

It may help prevent lifting of the covering fromthe wood, thus less wrinkles on a flly sheeted surface. But the only real weapn against wrinkling from heat is to heat the covering to the max shrinkage temperature while applying it. The temp is just a few degrees below melting.. when you heat the covering that hot, it takes a firmer set, and is more resistant to relaxing while the plane's sitting in the car.