Don't buy your first washboard and immediately take it to a session and beat the hell out of it. Less than brilliant percussion will wreck any session, and possibly lead to a lynching. Like any musical instrument, it takes a lot of skill to play it tastefully and effectively, in a way that enhances the music without interfering with the way the melodic instrument players can hear each other. You can only learn that skill by doing it alone or with a teacher for a long time.
The hardest part of playing any instrument properly is the rhythm hand - the one holding the plectrum or the bow or the fingerpicks. The washboard has two rhythm hands which makes it twice as hard.
Ray