3DPrinting

I've done this before on smaller parts and it works. The result is very ugly-looking in my case though.

I do this right regularly. I created a huge trash can shape container using 8 separate pieces printed in PLA. I then glued together with CA glue and welded the pieces with a 3D pen to make the joints stronger.

A couple of things to remember:

3D pen don’t push the filament like a 3D printer does. You have to slow the flow of the pen down, and let the filament bind to the layers below

There’s a big difference how different PLA will behave with a 3D pen. Some filaments won’t stick as well like others

You have to mimic the behaviour of a printer with the pen. So you have to lay down layers of filament, rather than trying to make it behave like a glue gun.

I've used a cheap 3D pen to weld smaller prints together. Works pretty well. You have to be quick because once the filament starts to cool it doesn't bind well. It also doesn't look that pretty, but I only use it in areas that aren't visible in the finished product. If you're planning on sanding it after I think you'd be fine.

I designed a part that has compartments for small neodymium magnets.

If the magnets are orientated with the layers you can always pause the print at the top of where the magnet needs to be embedded, just before the final bridging layer and insert the magnets mid print. Then resume the print and it should seal them in with a clean layer above it. Should be much cleaner then using a 3d pen to cover it up after the fact.

As for sticking parts together I can see it being useful for smaller or thinner parts, but for larger areas there are glues out there like gloop that can essentially melt and weld parts together more effectively with larger open times then you have with rapidly cooling pla.

I can see it being useful for spot repairs or filling holes or tacking parts together while you wait for glues to set.

For joining parts, you could also consider friction welding. I'm not sure how easy it would be to fill a cavity, but bond strength would be great since you would also be the heating the original part.

I have the MYNT pen you have pictured and I’ve used it a few times to patch up bad holes or other spots on prints that I did on my 3d printer.

It does the job when it comes to filling spots and am guessing it would easily do what you are describing. I know I find it a little unwieldy (I think that’s the right word) when it comes to making it do intricate things. I’ve seen pictures of detail work people claim they’ve done with them and meh - it’s beyond me. I’m decent with an airbrush but it doesn’t handle the same way.

I guess keep in mind - the pluses is that it uses the same filament you are using so it will match color wise.

The minuses is that it can be inconsistent with the flow rate and can be uneven with how it fills and looks so be prepared to (possibly) do a little extra sanding to get it level.