Well, you remember I said in a recent post that it’s essential to hold the board down while you level it? Actually I’ve had second thoughts about that. Basically it depends on how important it is that the board should hold its shape in the construction. If it’s fixed on all four corners and preferably in the middle as well, then you’ve saved wood by clamping it down. If it needs to be flat, i.e. not twisted at all, because it’s holding something else in place, then it’s better to let the board relax (perhaps hold it down in one corner or put a spacer under at least one corner so it doesn’t rock), flatten one side under the gantry then turn it over (at which point the side you’ve just done should sit flat on the table) and do the other side. Worth doing the ‘wrong’ side, if there is one, first. You’ll end up with a board that stays flat, as long as it’s not too thin by the time you’ve finished. That depends to a large extent on how well you glued them together in the first place, and of course whether you decided to risk putting a wonky board in between two others to persuade it to behave. Definitely worth dry-assembling edge-glued boards with biscuit joints for several weeks indoors before jointing and gluing. Also a good idea if you have multiple boards of the same length to have a symmetrical pattern of biscuit slots so you can interchange boards later if necessary.
Two more tips:
Make sure your cutter is sharp, otherwise you’ll be sanding out tiny gouges in the wood afterwards. I used a dish cutter because it has a flat bottom but curved sides, so any gaps or inaccuracies from the gantry are at least quite smooth and will sand out more easily.
Secondly, do the flattening in multiple passes, not all at once. This makes the pass easier on your muscles, the router and the router cutter, and reduces the risk of the router tipping, which makes a deeper cut at that one point. It also means that you won’t take too much off and find the board is too thin when you’re done. I found that increments of the thickness of a business card is about right. It takes twenty minutes to do one pass on an eight-foot by 19″ board; it’s worth the extra time to do several passes.
After the penultimate sanding, I also sprayed the board with water and wiped it down , then let it dry. That raises the grain, so the board feels rough. Another quick whizz with the ROS at 24o grit makes it silky smooth again.