Skirt steak and flank steak are ideally marinated for 4 to 12 hours. Skirt steak can be marinated overnight, or for up to 24 hours. Although you can marinate skirt steak for longer, it won’t improve the flavoring, and may break down the meat too much.
As steak marinades, the flavoring penetrates through the fibers of the meat. For tougher cuts, such as flank and skirt, this helps to break down these fibers and tenderize the meat. The loose knit fibers of the skirt steak makes it the ideal meat for marinating, and the strong beef flavor works well with a range of spices.
Acid is a vital ingredient in a marinade, as this is what breaks the fibers. When the meat is left to marinate for too long, it breaks these fibers too much. Rather than becoming tender, the meat turns mushy.