I've heard that the silicone sacs have some issues with gas permeability. And, I've been recommended to get PVC sacs from Woodbin for pens that may be prone to ambering and discoloration.
Si sacs do have gas permeability. But depending on the pen, it may or may not be an issue.
In correspondence with David Nishimura, we have come to the conclusion, that if you are going to use it in a clip pen that is stored nib UP, Si sac is fine. The ink in the sac prevents the air from diffusing into the sac. And because the nib is UP, the ink will not leak out of the pen.
HOWEVER, you do NOT want to put a Si sac into a desk pen that is stored nib DOWN. Because in that situation, the gas permeability will cause air to diffuse into the sac (above the ink), and cause the ink to leak down out of the pen, into the pen holder, creating a mess.
The other hassle is the cement. Si sealant dried MUCH slower than shellac. And you need to make sure that the sac is held onto the nipple until the Si sealant cures = HOURS.
So as FB said, for most of us, a latex sac is good enough, and that is what I use.
For a loose section, I do as you wrote.
I will put a THIN coat of shellac on the section, then let it dry for at least an hour, then test fit. If it is loose, I put another coat. I repeat this until the fit is snug. Note I mean snug, not TIGHT, as I do not want to stress the body. It just needs to be snug enough so that the section won't come out on its own, or be too loose that writing is a problem. This also makes it easy to pull the section when you need to change the sac later.
Edited by ac12, 22 November 2015 - 18:36.