I'm interested in these details as well.
I've been trying to get going on building a birchbark for 3 years now. I have reviewed and re-reviewed any resources I am aware of (see list here;
http://www.wcha.org/bbresource.php )
Apologies to experience builders who have dedicated their lives to learning these materials and techniques. I am beginningto realize it takes years to gain familiarity and proficiency with the materials. Hopefully you will have a good laugh at these ignorant newbie questions. And I understand any reticence at sharing the hard won and complex details.
Re prepping cedar (gunnels, ribs, sheathing) beforehand;
Apparently it isn't a problem, as long as it is well soaked before building. Stretching out the prep time may help save the body from ergonomic affects ie. pain but can also shorten the final assembly period.
I asked several builders if it would be a good idea to store bark rolls in a rain barrel until used. Apparently NOT! (due to possibility of algae affecting the appearance?) and apparently not necessary. I've seen a whole shed full of bark rolls that a builder intends to use. There was enough there that he obviously won't be getting to them in a while. (After my two years limited experience harvesting bark, I wonder where and when he got it all!)
Although I haven't had any experience with using long stored birchbark, I understand it softens and unrolls well with application of a little heat and moisture. Dip it in the lake, or soak it for how long before unrolling?
Re spruce gum;
I have been told to use "fresh" gum only. I presume that means gum that is still soft when collected, rather than that that has hardened like amber between seasons of leakage. I collect resin using a stick, into a coffee can. Once heated for straining, the amount of bark/needles/dirt/insects varies considerably as does the volume of strained resin. The colour is lighter when fresh gum and lower heat are used. If you want to really appreciate this natural material, dip a stick into it before it hardens and chew some. Once it cools, it's hard and brittle.
I asked a builder whether it is ok/preferable to add tempering fat* a the time of straining (which may be well before time of use) and got an inconclusive answer. By adding fat before you need it for use, you may attract animals to eat your stock. Might it also go rancid if not stored in a cool, dry place?
*Preferred fat varies widely, but includes, bear fat, Crisco, lard, peanut oil,....
I learned to store it in an old frying pan (have salvaged several of these from neighbourhood recycling bins, but also thrift shops). Note sawdust collects on the surface when stored open side up in the shop.
Re spruce root;
Instructions for preparation also vary considerably. Some say use fresh root. Some say boil immediately after collection for ease of removing the skin. Apparently root can be stored for some time if kept immersed in water, but I don't know how long it might remain supple/useable. Not sure if it can be resoftened for use if allowed to dry out.
And on it goes,...
