I've finished the south side tympanum (gable area), and a miserable task it was, with windy Winter weather and no scaffold from which to work. A close look at things up there revealed that all the shingles and much of the trim need to be replaced. 125 years in the blistering Pasadena sun have turned those shingles into tortilla chips; they are so brittle and warped that all I could do by way of surface preparation was to scrub each one carefully with a brass brush to remove the oxidized paint. The wood of the vent casing is quite fragile as well.
Because I need to keep this project moving along, and it took forever just to brush the whole thing by hand, I decided to skip the plastic surgery work for the time being; I can come back to it after the rest of the house is done, and I have longer, warmer days to work in. It's so far up there that it's hard to see the flaws anyway.