I put the two pieces together and hold them for 30 seconds. Now the paper sticks to the beam and is very resistant to lateral movement. It pops right off of you pull up on it, but to hold something down while you apply side to side force, it holds fast. Here's the beam with the 80 grit attached :
This might be hard to see, but I've drawn a squiggly line with a pencil on the edge of the board. When I sand I'll be able to see low spots.
After a bit of sanding, I can see where those spots are
I need to keep sanding until they start to come off.
Once they're gone on both boards, it should fit together.
You see the seam? Me neither.
I need better clamps. There will be a short wait while I buy them.
To be continued....