Why Prime?

I work with pallets and if I paint them I just paint twice. Sometimes I just sand off some of the paint and coners to make it look old. lol

 I would like to know what would be good on OSB boards. (My dad called them chip boards.) The side that looks like rough chip boards. What would be the right way to paint it? Hopefully make it a little waterproof so it will not swell up when it gets wet.  I want to make crafts and boxes with it but I don't want it falling apart on people.

Thanks