Jackie is totally correct about not getting the cardboard too wet, and to coat both sides of the cardboard.  It is better to coat each side with two very thin coats than one heavy one.

When something like cardboard or wood gets wet, the fibers swell and separate and surface bulges outward.  Then, as it dries, the surfaces shrink and warp.  But there is much less chance of that if you treat both surfaces the same way.

And there is no such thing as a silly question!  If you don't know the answer, you have to ask a question, and never let anyone tell you different.

Sue