Not exactly correct. Grape juice may or may not increase body. After all, grape wines may be high or low in body. Grape juice is added it improve the balance (hopefully) and the vinuosity of the wine. What is vinuosity? Well it means the wine is more wine like. Real clear, huh!
As far as using supermarket juice, it depends on the juice. Some grapes may make a poor wine and you don't necessarily want that. Here in the states, concord grapes are used for most red or purple grape juice. Some people like the taste of concord wine, many don't. It is a very powerful taste and it can dominate the other fruit if you use too much. Niagara is common for most white grape juice and it blends very well with fruit.
mold on wooden press slatsRob not a fun situation there. If the wood slats are varnished, then surface scrub with potash (potbuttium hydroxide) will probably be enough...
My suggestion is to make a small batch of wine from nothing but the store-bought juice and see how it comes out. Then see how it blends with your fruit wines. Then decide if it will work. Visit Jack Keller's site for recipes using frozen store bought grape concentrate.
Incidentally, the most accepted fruit for improving body is banana. It does not seem to impart much flavor on it's own but really boosts the body. Use cheap overripe bananas that the store is discounting.
Ray
