I am making wine from black homburg grapes and the primary fermentation has completed (10days). I didn't have a hydrometer at the start, but have transferred the juice from the must to 2 1/2 demijohns and noticed there is no more fermentation, so I then found the sg was about 994 and realised I should have added sugar at the start of primary. How can I calculate how much sugar to add at this stage? Should I go back to primary fermentation in an open container with loose cover or keep it in the demijohns? Any help would be greatly appreciated.
IMHO for this situation it is best to leave the wine as it is and bottle when appropriate.
If one does not know the O.G. of the brew then it is very difficult to guess what it was and thus to attempt to take any corrective action. If the grapes were ripe and well formed then they may well have had enough sugar to make reasonable wine - or they may have been sugar deficient. How could one tell? It is very difficult if not impossible. Obviously there was a sufficient amount of sugar originally present to produce enough alcohol to the F.G down to 0.994. So that is a good sign. As this is not a great amount (about 5L/1gal?) I would recommend leaving it to age, bottle it then leave it for a year and see what the natural juice fermentation produced.
If one really wanted to experiment then one could put another 25g of sugar in one demi-john and see how that went. The only problems with such a scheme is the yeast may have reached its attenuation point (so adding more sugar will only make the wine sweeter) or the use of unsterilised sugar my introduce some other microbe that may act to affect the taste of the wine. However at what point would one stop adding the sugar? Its a moot point... perhaps best to put this down to experience/learning and remember next time!