I haven't had a process black in stock in ages either. I do use either FrogSkin or Gerber's UV Abrasion Guard on anything that is expected to last long term outdoors.
Process Black is slightly less opaque than Spot Black, but for F/C prints, subbing Spot Black doesn't change the print much, so it's easier just to have the one foil in stock.
The only time I wouldn't sub Spot Black for Process is if I'm using SpectraTone colors because then the shade may turn slightly different.