On spruce , filling is a waist or material. There are no pore for it to fill .
The process for filling is as much technique as it is material. I have used so many fillers in my building history and here is what I learned
A SEAL FIRST
if you don't the filler will shrink , the object is to seal the wood , that will help block the pores so the filler will work better
B NOT ALL FILLERS ARE THE SAME
I have used pore o paq from behlen for a long time and have very good success with this product. I would get it in natural and stain it for each application.
my new favorite one is the LMI water based filler. It has ample work time , and does a great job. You can rub it in by hand and unlike other water based fillers it doesn't dry so fast as to make a mess of the job.
C EGG WHITE AND SANDING
Yes this does work the protien in the egg white will adhere with the dust and fill pores.
The best advice I can give is #1 READ THE LABLE #2 Sample the product and your technique.
Martin taught me my process and I do pretty much what they do. Apply let flash and rub it with a wool bonnet on a hand help buffer or hand rub with denium or something rough like burlap.
I do this at least 2 times
so
STAIN ( minwax brand is also a sealing stain ) I still shoot a coat of sealer over it.
allow a few hr to dry depending on product. Apply filler then seal
If I am using Nitro I hit the sealer in about 10 min with a wash coat of nitro as there is no need to sand a seal coat.
apply finish per instructions or your technique.
learning to do a good finish is often harder than building the guitar.
_________________
John Hall
Blues Creek Guitars Inc
Authorized CF Martin Repair Center
president of Association of Stringed Instrument Artisans
http://www.bluescreekguitars.com