Depending on the amount of marinade, there are a few options:
1) If there is a little bit of extra marinade (or if you are just sealing some juicy stuff), you can put a bit of paper towel and the end of the bag. When the liquid starts getting sucked out, the towel will absorb it and allow the bag to seal. This is a good idea anytime you seal anything with a lot of moisture.
2) Most of the sealers have a quick seal feature, where you can cause the bag to seal before all the air is removed. This is good when you seal pure liquid (like storing sauces etc).
3) Finally, the double bag approach. Put your items and liquid in a ziplock bag - the freezer bags work best since they have a very firm seal. Take out as mush ir as you want manually, then drop that bag in the vacuum sealer bag. This one works well with pickled type items, where you want to seal an item in a lot of liquid, and be able to open it without spilling stuff all over the place. Also, it won't overmarinate an item. If you were to vacuum seal somethign porous without the ziplock, a lot more marinade would be drawn into the item.
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