Well now that we've got all our lumber all cut to size, we're going to go over the lumber and look for any defects, knots, things like that, that are little gaps in the wood and stuff and fill them with some of this tan colored filler. You want to use the tan color and not the white because the white is going to show up. You can use the white if you're going to paint the bookcase, but in this case we're not going to, so we're going to use this tan stuff. When you're filling a crack the best way to fill it is to go across the crack not lengthwise. Now that we have that done, do that to every piece of wood and set on side until you have all of your wood filled and ready to be sanded. When we're done doing that, we're going to sand the wood. I'm going to use a belt sander on this, but you can also use a palm sander if you want to. This is a one twenty grit belt. If you use a palm sander you're going to need to use one hundred grit paper. And we'll start our sanding.