That’s a great question with a very simple answer. You should not add postassium metabisulfite (SO2, or sulfur dioxide) to your wine between primary and secondary fermentation. Because the SO2 will inhibit or could completely kill off the bacteria you need to conduct the secondary fermentation, it’s critical that we never add SO2 to wine at this time.
Malolactic bacteria are very sensitive to sulfur dioxide and your malolactic fermentation (aka “secondary fermentation”) could be completely inhibited. You may add a little bit of SO2 (up to 30 ppm) when you crush, to ward off spoilage organisms that might be antagonistic to the yeast you want to conduct your primary fermentation. But by the time the wine is fermented until it’s dry, this sulfur dioxide will be mostly, if not completely, in the bound form and not able to interact with any malolactic bacteria you might be adding to conduct your secondary fermentation.
There’s no reason for you to wait between the end of primary fermentation and inoculating for your malolactic fermentation. If you must wait for some reason, however, you should be ok if you drop the lid on your variable tank down on the wine surface to exclude air. The residual carbon dioxide gas in the fermentation should help keep the wine protected. Inoculate with your malolactic bacteria as soon as you can, though, and be sure you keep the next SO2 addition until after your malolactic fermentation is complete.
