> Still you get the occasional missed pin you need to re-solder but it works.

How? Did you mean to remove all the existing solder, reapply the paste, and reflow again (can that even be done on an already populated board, where do you place the mask/stencil?), or it's something else?

More of just putting a bead of solder on a soldering iron tip and fixing that pin (best with a microscope setup). If you get the technique just so, you can do a form of "drag soldering" to fix a missing pin or two. Otherwise you can do a quick dab with a bead of solder since with most QFN's you can still see the side of the pin but you may need to rotate the board. I think there are a bunch of great youtube videos on doing that.

Getting a proper solders for QFN's really does best with a proper metal stencil. If you use JCL PCB, use their "electropolishing" option otherwise on fine pins the paste sticks more.

Removing and reflowing the part is tricky and often damages the part. Also to not ruin other parts of the board, a heat gun is often easier. We've never really tried reflowing parts unless a board didn't "bake" properly.

Confirm you meant to say stencil rather than solder mask there? (Solder mask is the colored material over the board, while a stencil is a separate piece of metal with holes cut to allow solder paste application with a squeegee.)

> Confirm you meant to say stencil rather than solder mask there?

Yes, stencil is what I meant to say. The "mask" part of solder mask seems to throw me everytime I mean to say stencil, as in I want to mask the parts of the board _not_ to solder. Someways I wish English were more like Swiss/German or Hebrew.