If "grain filler" didn't mean anything to him, I doubt much of that post did.
OK, I don't know anything about finishes, but I know what grain filler is, so I'll describe it:
If you look at your wood, depending on the kind of wood it is, there will be little "holes" (or "pores" if you have a Swedish-to-English dictionary) in it. If you just try to paint over top of that, you will always be able to see little bumps in your paint job.
SO, people fill in the little holes with a special substance/putty/glue called "grain filler" which makes the surface flat, and elminates the little holes. If you go to a wood/hardware store, they will know what you are talking about. Some people on Project guitar do not use traditional "grain filler" because it actually changes size over time, shrinking a bit. There are a variety of things our members use, including CA, which I don't know what that stands for, but it's basically a type of glue.
Hope that helps a bit. Somebody else feel free to correct what I've said and provide missing details.
Greg