Many houses have hollow-core interior doors-thin pieces of wood veneer mounted on either side of a wood-strip frame. They're inexpensive, which is why they're so common, but if you have the chance to replace them with solid-core doors, go for it. Solid doors are much sturdier, and they also offer better sound, heat, and fire resistance.
In the meantime, however, the most common problem with hollow-core doors is that they're vulnerable to getting holes knocked through them. You can repair this similarly to a hole in a wallboard wall (see Chapter 13). In this case, however, you'd create a backing for the layers of joint compound by spraying expanding-foam insulation into the hole or by gluing a mesh screen against the inside of the door (a sturdy thread woven into the mesh on two sides of the screen allows you to hold the screen in place while the glue dries).
However, you'll then have to completely refinish the door in order to hide the repair, and even after refinishing, you'll probably be able to tell where the repair was made. It may actually be faster to replace the door. Measure the existing door's height, width, and depth, and the locations of the hinge plates and the door handle. If you buy a new door slab you may have to cut mortises for the hinge plates and the latch plate, and two holes-one for the door handle mechanism that goes through the door and the other for the bolt that goes through the door edge. (Check the sections on installing deadbolts in Chapter 10.)
Alternatively, check out reclamation or second-hand building material stores to find a door that will fit your frame, which has hinge mortises and the door handle in the right locations. You might well get lucky, because many doors have the hinges and handle in the same place.
To fix a scratch, buy a wood repair pencil that matches the door's finish; run it along the scratch, blending it with the existing finish. For deep scratches or large areas of damage, you may need to use paintable wood putty or an epoxy wood filler that you can finish to match the existing surface.