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I am currently building an OM with back and sides in black limb. I jointed the back when it was in the square and 4.5mm thick. I then proceeded to rough cut the shape out, thin it to 2.5mm, add an internal centre reinforcement seam and the four braces. I then realised I had made a cock up and had to take it all apart again. Using heat to take the braces off has resulted in the centre joint separating too! Now I have to rejoin the back halves that are only 2.5mm thick and are guitar shaped!
What's the best way of doing this? (I did the original joint using the tent method)

Many thanks for any advice you can offer to this poor dumb arse!

It can be done, no problem. Firstly you need a straight, flat piece of backing wood the full length of the joint. Something fairly substantial - probably 1 " thick at least, perhaps 2 " wide . If you have some long reach clamps (Klemsia) you can just use another piece of similar wood for the top surface of the joint. Glue and pull the joint together with the tape and then clamp down with your flat pieces of wood.
If you haven't got long reach clamps you might have to make a curved caul - running the full length of the joint.

I do this with a large flat board a couple inches longer and wider than the back to be joined (5/8-3/4" plywood is fine), some weights, waxed paper, and those large gun metal colored tacks you can get at the hardware store.
Of course the waxed paper goes between the wood being joined and the board. the weights provide the down pressure to keep the joint from popping up as you apply side pressure with the tacks.
The key to this is that the tacks have a "V" taper from head to tip so if you set them in with the tip up hard against the edge of your wood, as you drive them in (straight down), the taper supplies side pressure. Also when driven in to where the head contacts the wood they will hold everything flat against the backboard.
Concentrate on the side portions for most of the tacks (10 or so per side) as well as a few top and bottom near the join to help hold things down.....use the weights for the center portion along the joint.
One can join any sort of profiled shape with this method.