Topping up for quality

 At the end of fermentation, the wine should be separated from the lees (sediment) and stored in a fresh vessel. If you are working with 5-litre bottles, typically, between sedimentation and volume loss, you will be able to transfer about 4½ litres, leaving a ½-litre air space above the wine. This is too much air and would impair the final quality if you simply sealed the bottle for storage. Fortunately, there are several options for topping up, some better than others, but the choice is yours:

1. Water. This is the simplest approach but of course it dilutes the wine, reducing alcohol by volume, acidity, tannin content, intensity of colour, flavour and aroma, and body. This sounds bad but some wines can benefit from judicious dilution.
2. Wine. Topping up new wine with wine from an earlier brew is often the best answer. Obviously, the added wine must be sound. Normally it should be of similar style to new wine, though judicious corrective blending is possible, for example, to increase acidity if the new wine is lacking in that department.
3. Spirits. White wines can be topped up with a neutral spirit like vodka. Red wines can benefit from an addition of brandy. This is an expensive option to be used in exceptional cases only. For example, a white wine at 13% with residual sweetness can be fortified to 15% to preserve the remaining sugar against further fermentation, producing a strong desert-style wine. Only consider this option for wines that are showing early signs of high quality, and be prepared for a long aging process.
4. Diluted spirits. For a table wine that is already the strength you want, e.g. 12%, you can top up with a 12% ABV vodka/water mix. This still dilutes all of the wine's characteristics just as water does, but it preserves the alcohol content.

In summary, Option 2 is usually the best option. Option 1 is cheap and cheerful, Option 4 is good for lighter wines, and Option 3 is for specialised purposes only.

You'll note there is no fruit juice option. Anything that adds sugar at the top-up stage is to be avoided as it is asking for trouble – flying corks, burst bottles, permanent hazes... Don't go there.