My question pertains specifically to the top of the primary fermenter. Do you keep the top off the entire time you are racking or will that risk too much exposure to oxygen? Racking from a plastic bucket to a glass carboy. Thanks for any advice!
I can't see it makes any difference. The headspace is going to fill with air anyway, regardless of whether the lid is on or not. If you can do a closed transfer and fill that headspace with co2 that is best, but otherwise just do the transfer as best you can with minimal splashing.
This is what I did for a bunch of years with no trouble. I left the top ajar if using a racking cane. If the primary has a valve I just pulled the airlock out. I would purge the secondary with CO2 (if you have it) before filling it.
Hope this helps.