A couple of years ago I bought a Framer's Corner PFK04 frame joiner and wrote a review of the initial experience here. I've now made several hundred frames and I'm pretty good at it and I thought it was time for an update of my review.
You can see the main bits of equipment in the picture above. The PFK04 is at the back, with its clamp at rear left. In front is a frame assembled and setting in my Bessey strap clamp. Finally there is the bottle of PVA glue, a little jar of this glue diluted to 50:50 with water, and I have a few small paint brushes to apply the glue.
After trying different processes, I've standardised on assembly with V-nails first, then putting the finished frame in the strap clamp. Initially I tried the other way round but this was too difficult.
The first job is to "size" the ends of the moulding with 50:50 diluted glue/water. This dilute mix is absorbed into the end grain and seals it so the full strength glue will then sit on top. Otherwise I think the danger is that the glue gets sucked into the grain while setting in the clamp and weakening the joint. So I paint on a very small amount of 50:50 to each end surface and leave for a couple of minutes. Then I paint on the full strength glue to two matching ends and join up in the clamp. I've learned to judge the correct amount so that a tiny amount squeezes out but no more, ensuring complete coverage of the joint. If the glue doesn't extend completely to every edge then the unglued edge will be a stress raiser and a weak point.
The U-shaped piece of soft wood the clamp is sitting on was mentioned in the original post. Because the clamping rod projects underneath the clamp it's awkward to operate the clamp with it sitting flat on the bench. The U-shaped thingummy raises the clamp off the bench so it doesn't wriggle while I'm using it.
The piece of scrap card on the joiner is there because I found the surface of the joiner, which is rubberised, to be a little too grippy when I wanted to slide the clamp around on it.
I now get good and reliable results, though to this day I still don't attempt stacking the V-nails. Using a few accidentally damaged frames I have tested them to destruction to see how strong the end result is and I'm very happy. It takes considerable force to break them down, far more than any frame would expect in normal service.
I still only use a very limited range of mouldings, from Lion. I've found that the quality of the joint, in terms of flatness and flushness, depends a great deal on the straightness of the moulding. Occasionally the supplier will send pieces that are warped and I've had to discard a handful of frames as a result but my success rate is now very high. My small size prints are mounted in a 300 x 400 mm mount and I order cut glass in this same size. When I order the mouldings I add 4mm in both directions, to give a 304 x 404 mm frame. The moulding supplier normally adds an extra 2mm tolerance to this but they let you specify not to include this. The 4mm tolerance is essential to allow for slight variations in the size of the mount and the glass but also so you can get the mount and glass in and out. My larger frames have the same +4mm tolerance.