I actually only planed the edges and put up with a bit of twist. When the slats were glued together, the edges glued well but the faces had a bit of up and down (if you know what I mean). If you wanted to prepare the timber correctly and fully, the first step would be to run the face of the board over the jointer (sometimes called a surface planer). When the wide face of the board is perfectly flat, that flat face is then placed against the fence of the jointer and one edge is then prepared. This edge will be flat and also at 90 degrees to the face. The board can then be ripped on the saw bench to the correct width. Finally, the board can be ripped to the correct thickness or run through a planer/ thicknesser. That will give you a board that can then be attacked with a router and T &G bits. You would need to follow a procedure similar to this to ensure that your material is true before trying to use joint work. If I had a pallet that had thicker slats, I could have done that and it would make life easier as all components would fit together well. The problem with using pallets for this type of work is that the majority of the material is of poor quality and the slats are normally green, unseasoned pine. If you live in a dry area where you can store the pallet for some time, it will dry to a moisture content that will allow it to be used. On this project, I saw that the main bearers were kiln dried so I cut the pine and glued it up for the critical parts (top, base and nuc ends). I decided to use the slats for the sides of the box as a bit of unevenness could be tolerated. When I first started keeping a few bees, I made some frames out of pallet material. That was a big mistake. Some of them warped and twisted in every conceivable direction. I won?t be doing that again.