The only exterior work I do now are sash windows - post refurb - where we remove them, prep and paint as component parts then reassemble.
If its small fills then I use standard 2 part (Tetrion 2PK, Ronseal etc)
For fills above say 20mm I use Repair Care or Metolux. For sash windows we use Repair Care on construction joints as it not only fills but binds the two pieces of wood together forming a watertight seal (Pics of some repairs I did are on the Repair Care website)
Toupret Fibracryl in tube is superb filler too for wood to masonry joins
For fine surface finishing Toupret blue tub Fine Surface Filler