slate
ive always damped wiped my slate before putting on new cloth. cleans off all the fine dust and any repair debris. never a problem. over the years of use i even damp wipe my cloth to remove chalk dust and house dust. it evaporates rapidly. just think of it as running an iron over silk. dont press down really hard, just enough to rub the cloth surface. as for the slate, doesnt matter how hard you press. thats my opinion having done this for nearly ten years without problems to slate or cloth