| He has his own brushes. He doesn't want to use them on oil-based paint. As
| I wrote, he works interior. No one around here uses oil-based paint in the
| interior. And many of the houses here have unpainted woodwork. So he is
| mostly restoring plaster. These 120 year old houses settle and the walls
| get cracked.
|

Vic Smith makes a good point. Anyone with decades
of experience should know what he's doing -- in or out,
acrylic or oil. Patching failing plaster is a very minor
skill that most painters know. It' not a trade. It sounds
like your helper is either not very bright or is a plasterer
(doing new walls with blueboard) who occasionally does
other odd jobs.

It's up to you if you want to use unskilled workers
and try to plan the job yourself, in order to save money,
but I don't see why you're representing your worker as
an "old master". However many years he's been working,
he's clearly not very skilled. If he were you wouldn't be
here asking for advice because he would never agree to
do the work according to what you were told by a bunch
of wiseacres online. Nor would he agree to be your
employee by the hour, which seems to be the arrangement.

A separate note: If you have your mind set on using junk
brushes you can make that more efficient by freezing them
overnight. I usually do that with any rollers or junk brushes
I'm using and don't intend to clean. Oil in the freezer. Acrylic
in the fridge. Acrylic rollers will last for days, at least, so
that I can use just one for each wall color and keep it for
the duration of a job. Oil rollers and junk brushes in the freezer
will last for weeks. In both cases I just wrap them in aluminum
foil or put them in a plastic bag.