The whole pipe should be sanded down with 60 grit paper. Use long strokes parallel with the length of the pipe. The pic here shows sanding before it is even cemented. It's probably best to just wait until the whole thing is cemented, heated, bent and filed. Make sure you sand off the lettering on the pipe, they could show through depending on the shade of stain you use. Rough it up pretty good so the primer can stick to the surface real well.
The next step is to primer it. What I do is pound a stake in the ground and slip the didj over it to stand it up verticle, hands free. Then I just use a white spray can primer for plastics and coat the whole thing. It may take a couple of coats, especially if you are using the black ABS. Let dry the recommended amount of time specified on the can.
Now you are ready to stain. Remember, this is not wood so do not try to rub the stain into the surface. It will only remove your primer if you do so. Use your favorite wood stain, dark colors seem to work best, and liberally wipe it on with a rag. I start at the bottom and work up. Once the whole thing is coated I take and make long wipes with the rag that go all the way from the bottom to the top without lifting the rag. You may have to dip the rag in to the stain a few more times as well. This will produce 'wood grain' into the surface of the didj. Let this dry completely, 24 hours or more. Once again do NOT go back and try to rub it in like you would with real wood. Just let it dry.
The pic on the right shows three of my synthetic didjs. (no, the pic is not upside down) The one in the middle is the one in all the pictures of this tutorial. I have since painted more designs on it but this pic shows the stain pretty well. A closer view would reveal more 'grain' than you can see here. I finished it with Red Mahogoney stain.
Construction of the PVC/ABS Didjeridu