I missed the beginning of this thread, so I am not sure what is the
need for thrumming.
Different areas require different approach. I never use commercial
thread of any kind. Thrumming usually is a next step after piercing.
A lot of time it can eliminated by piercing carefully and using an
old blade. If super-fine finish is required, I recommend 2 steps
Flatten copper wire, insert it into saw frame ( flexible frame works
better for this ) and use it with some tripoli and oil. Watch out, it
can cut as a saw.
Second step is a bit involved, but sometimes needs to be done. Double
the length of silk thread and wax it with beeswax very deliberately.
Find some old piece of fur coat and give it a haircut. Collect the
hairs and distribute small amount between waxed strands. Hair should
lie perpendicular to the strands. Some of it will fall, which is ok.
The ones that adhere is enough. Start twisting the stands of silk.
The twisting entraps the hair and that makes a very good thrumming
thread, which adapts to any shape of opening.
Do not expect good results right away. Thrumming is like burnishing
requires a lot of experience.