Well, the jointing idea worked, thank goodness. The idea is that you’re not relying on each edge being perfectly straight, you just want the two edges to match each other, so you cut both edges at the same time. That way it doesn’t matter if the guide isn’t perfectly straight. I did, however, try to get it as straight as possible using a piece of thread stretched along it before tightening up the join between the two pieces of the guide.
I was casting around for a suitable spacer for the boards; it makes sense to have them close enough to cut both edges, but sufficiently apart to cut off the minimum amount of wood. The kerf of the saw blade is 2.2mm, and I found that a lolly stick is 2mm thick, perfect. Then I found that the lolly stick has a nasty habit of falling between the boards which makes it awkward to fish out again, so the solution was to drill a tiny hole and fit a piece of wire as a cross-piece.
I’m managing to joint one board per day – that’s three joints per board. Seems slow, but there are several steps in the process: first check that the joint needs adjustment – about two-thirds of them have so far – then set up the saw guide with the lolly sticks and lots of clamps to hold the boards flat on the table, do the cut, undo the clamps and check that it’s OK, if not do it again. Then the guide has to be manhandled somewhere (I’ve cleared a high shelf) while I re-do the biscuit slots which again needs lots of clamping to make sure the board is flat on the table. Then test again and finally set up for gluing, and leave the clamps on for at least half an hour before moving it aside for the next one.
On the subject of gluing, I’ve discovered silicone brushes. Work a treat and easy to clean. I’m using Titebond III on this job.
All the 6-foot boards are done now, I’m halfway through doing the first eight-foot one. I’m glad I tested the concept on the shorter boards first.
And now I’m thinking about flattening the boards. Never had this problem with plywood… I’ve got a belt sander and a random orbital for going up the grits, but I think I’m going to try starting with a jig for the hand router to skim off the top before sanding. Watch this space.