I added sugar to my wine after it had been fermenting for 2 days. It had a reaction and started foaming up and out of the container. I lost maybe a half gallon of wine. I put the lid back on it and it continued to ferment. But what did I do wrong?
Dry powdered or granulated additions give the CO2 somewhere to come out of solution. You put in thousands of tiny points for CO2 to form bubbles. The CO2 is in solution from fermentation, to the point of saturation. So this comes out as a volcano.
Personally I would put all the ingredients in at the start, so they are al treated with the potassium metabisulphate. If you have to add later, you can make a simple syrup by boiling a small amount of water and dissolve the sugar. This sanitizes the sugar and it will not erupt while adding a liquid. Don't boil enzymes though, it will destroy them.
Do your primary fermentation in a brew bucket with plenty of extra room. Cover it with a dish towel rather than a lid.
I add all my ingredients at the beginning. But if you are going for a high ABV wine then you might need to step feed the sugar. In that case, I would dissolve the sugar in a small amount of wine or water first, then add it to the primary.