Compression Damage?

I have not personally tried this but many have recommended this method on this site. First, insert a waterproof pack liner into your bag. Next, push your bag (not in a stuff sack) down to the bottom of liner. Finally, put all the rest of your gear into the liner. The gear on top of the bag will keep it relatively compressed but not so much that it damages the bag (like it will in a compression sack). Because the bag is in the bottom of your backpack, it will fill in all the empty space/corners that are typically wasted. Some have reported that their sleeping bag takes up less space using this method than when they used a compression sack. Especially since you are planning a long thru-hike, do NOT use a compression sack if you want to make sure your bag has a long, productive life.

I might use a compression stuff sack for my clothes bag. Never for any high loft insulating gear.

A 4 L sack sounds far too small to me. I use a 13L dry bag (also Sea-to-Summit but without the "eVac" feature) for my sleeping bag. I try to leave the sleeping bag in there as short a time as possible (i.e. I pack it just before taking the tent down and unpack it as soon as the tent is up).

I've been compressing my synthetic summer bag each camping trip for the past 5 years. It's lost some of its fluff, but the difference in insulative properties probably isn't consequential given the climate it gets used in. I wouldn't want to do the same with a nice, volumeous down bag, though.

I too only keep a bag stuffed as long as it needs to be; sometimes I'll even unpack it and let it air out during long lunch breaks.

I do the same thing as John with my synthetic bag.

Me, too, Mary D. Minimum time, minimally compressed = more warm nights in the sack.