There are several questions here - I'll take a stab at some of them.
I tried using bentonite after racking with good success, going against the advice of 2 local wine kit suppliers that told me to add it up front. This time I decided to see what would happen if I added it up front (it's only been 2 weeks) and I can't think of a good reason to rack it. If I rack it, will I lose the effect of the bentonite? Do I need to add more?
Maybe I'm not understanding but if you are using a conical you shouldn't need to rack. For folk using carboys, they can continue to swirl the container to resuspend the bentonite a few times to make sure it binds everything possible, and then wait at least 24 hours for the heavy stuff to settle out before racking (I usually wait until it is a clear as it is going to get before racking.
After racking there may still be some residual bentonite in the solution, especially if it is not clear. It will continue to settle after racking. If you keep adding bentonite it will strip the color and flavor from your mead. In wine making it is generally recommended not to exceed about 0.5 g/L.
When I added it up front there was a 'conveyer' effect - where particles of bentonite float up and down for a few days early on, and then settle out (I don't know if this is why I was advised to use it early.)
Probably. In very clear musts, bentonite may improve fermentation kinetics, but in wine, it also increase H2S production. I'm not sure if it occurs the same way in mead, but keep alert for the possibility.
A big negative is that using it during fermentation in wine making means that you cannot age the wine on the lees as the bentonite treated lees will produce off-aromas. Again, I cannot tell you for sure if the same thing will occur with mead, but if anyone has tried aging on bentonite fermented lees please chime in.
but I worried how much it would expose the brew to oxygen? I'll have to open the lid to swirl it in. Maybe I'm more worried about exposure to oxygen than I should be.
After fermentation completes, there is usually enough dissolved CO2 to chase air out of containers so that opening them to gently mix in fining agents does not cause oxidation. But clearly, after about the half-way point of fermentation, you want to protect your mead from oxygen.
I'm not sure if I'll lose the effect of bentonite added up front after dumping lees because I'm not really sure how bentonite works. Does it work best during primary or secondary fermentation - or does it even matter, so long as you add it and give enough time to be exposed to your mead to clear before bottling(and/or bulk aging)?
It can work in primary or secondary. The advantage of using it at the start of fermentation, in addition to the kinetic improvement is that it may require less total bentonite.
The
Improved Wine making site has a nice article on fining agents - it is worth a read and will explain more about bentonite and how it works. Keep in mind that bentonite will not clear every mead, and you may need other agents at times.
After all this, again I would say that with fining agents you may sacrifice some color, flavor and aroma. I do not like to do this. Most meads will become crystal clear given enough time to settle which may be months. I don't even think about fining anything until it has sat for at least 6-9 months. You might want to try a batch without bentonite to see what happens. I'll give you another tidbit to consider - a little bit of oak may speed clearing as well as improving the flavor of traditional meads.
In mead making the most important ingredient (and the most difficult to find) is patience.
Good Mazing!
Medsen the loquacious