I just finished replacing all the hammer springs in an Aeolian with
blue felt.  I've had problems with weak springs in the past and so when
I saw about a dozen of them already broken, I went ahead and replaced
them all, rather than use repair springs.  Aeolian springs from this
era have been so bad, even from the start, that one could not bend them
to adjust hammer return speed.

As for whether or not the felt had anything to do with breaking
springs, I think not.  These were breaking outside of the coil spring,
at a point not in contact with felt.  There was no discoloring of the
felt that might indicate a reaction between felt and spring.  Also,
I learned early in my career that Aeolians had easily breaking springs.

Here's a trick I learned from my mentor years ago.  Use a straight rod
(a coat hanger will do) and run it through all the coils after you've
inserted them in their holes in the rail.  It will prevent the coils
from rolling over 90 degrees as you snug up the spring wires into their
slots on the back of the rail.

It's not a major job.  I was done in under three hours, start to finish.
I rarely have to do this job but it's almost always been Aeolians from
the 1970's.

Jeff Davis in Seattle