I don't think a nail clipper file will work. Not coarse enough. Don't use sandpaper, masonry block or Dremel grinding stones.... u just adding sand to the metal filings.
I don't do this with lead based solder. I'd want a full face mask and have to bathe after etc...lead powder ain't fun.
I did some basic solderability tests on aluminium foil to check the beading...it's ok, decent coalescing.
My mix has about a 25% flux content...which is a bit high...but u get excellent tack and wettability for lead free.
I get smooth silver looking fillets....not as crusty as with wire solder.
I am using my DIY HOT air pencil.
https://www.electro-tech-online.com/threads/diy-hot-air-pencil-8-easy-steps.113697/
Edit...
I used a dremel Burr bit and it makes quick work of the grinding, although the filings are not as fine as hand filing. Did some manual work with it...slapped on a bridging blob over both 805 pads. Melted with the Hot air...it coalesced nicely on both pads...no bridging. Rested the 805 part with a tweezerr and reflowed...perfect. Desoldered and reflowed the same part 4 times...no problem ..no damage to part or traces!
I added the fresh powder to the existing mix to up the metal content to about 85% as per a standard solder paste mix. Not as tacky as before though, I figure it would work for manual use here but not so well for stencil.
BTW RB, I really liked your 'black' page, there are several useful projects there I'd like to try. Would any magnetic sounder work with your 1 bit audio encoder? To play xmas music perhaps?