Originally posted by Tippie
What is probably happening is that the pointed edge of the triangle stock is 'bottoming out' at the joint of the firewall and the fuse side before you get good side surface contact. Take a razor plane and slightly trim the point of the triangle stock or just sand off the sharp point a little where it goes into the corner and you will have better bond with the sides/firewall. The reason is that there may be glue in the corner of the joint from when you bonded the firewall to the fuse and that will hold the triangle stock out away from where you want it. (If you can visualize what I am talking about.) Same goes for triangular gussets, sometimes your joint isn't a perfect 90 degrees, just clip a little off of the point of the gusset and it will fit better. If you are using epoxy or something like titebond the glues will fill in the slight gap left from trimming the point off.
Ditto what tippie said. I sand off the apex of the triangular stock a little to accomodate the pre-existing glue fillet at the corner of the "L" joint. You can use clamps for the firewall area and elsewhere but it needs some "creative" thinking. For my 4*60 firewall, I cut a small bar out of thick balsa, bevel the ends a little so that it can come into good contact with the middle surfaces of the left and right triangular stocks. Then I use a hobby clamp between that bar and the front side of the firewall. Both triangular stocks were well clamped with no gaps. I also have a variety of adjustable clamps : spring clamps, g-clamps, u-clamps, l-clamps, there's also one with CF rod (can't remember the make), cloth pegs, etc that comes in handy for different applications. Hope this helps.........