Feather Fill was a final finish filler used. I think there was a lot of pin holes. Later in life, I think everyone migrated over to Mortons Eliminator, This was a nice product as it was a primer/filler.
Today, I'm using a high build primer, no pin holes at all.
As for filling:
I use West systems for filling.
I also use glass ballons (The same stuff for making micro)
MIx it up just like micro, then add some of the West 410 Microlight Fairing Filler powder to it and mix it in.
The West 410 Microlight Fairing Filler significantly improves the way the micro flows.
I guess you could use the West 410 by itself, but it is rather expensive.
Waiter