Can you reuse wine skins? Just wondering if I could save some skins if I get wine grapes come fall. Of course they’d be fermented once.
The short answer is "yes". However, you may or may not get any benefit from re-use.
Second run wines are made from the pomace of grape wines -- the general formula is for every 2 gallons of 1st run wine, add 1 gallon water, 2 lbs sugar, plus acid, tannin, etc. The 2nd run can be a good wine, although lower in body, color, and flavor than the 1st run. This is dependent on how hard the 1st run pomace was pressed, as the more it was pressed, the less "stuff" is left in the pomace for the 2nd run. Keep in mind that this uses double the pomace to produce a lesser wine. IMO it's worth the effort, but it's important to realize it's not a 1st run wine.
If you do EM with skin packs, there will be less "stuff" left.
In a semi-related experiment, I used 6 oz oak cubes in 54 liters (14.25 US gallons) wine for 3 months. Removed the cubes, dried them, and sulfited before my son used all 6 oz in a 23 liter batch (normal is 1.5 to 2 oz). After 3 months we tested it, and there was oak flavoring, but our subjective tasting was that it was about half of what we'd expect from 2 oz fresh cubes. 3 times as much got half the result, so our reasoning was the cubes had about 1/6 goodness left in them. It was a good experiment, but we'd not bother re-using cubes again.
I wouldn't bother trying to re-use a skin pack, as what I'd get from it may not be worth the effort. IF I did try it, I'd drain the packs after the first use and immediately freeze. Before use, defrost in room temperature water in a sink (I assume the skins are in a plastic bag) and add to the must when mostly defrosted. If you try this, post your results.