If you are going to own a used or new military surplus tent or a surplus army tent, then you should have the facility to keep it in good working order. The good news is that the military had this in mind when they designed this thing, and they offer an excellent tent repair kit that you can get fairly reasonably to keep with you as you go about your military surplus camping business.
Here is an overview of the military issue repair kit. An alternative idea to cotton twine and polyester string is heavy duty waxed string, some people call it cat-gut string. This stuff is amazing, and some people even use it for saddles and other leatherworking projects because the wax offers a sealant to needle holes when drawn tight and holds a tighter grip than non-waxed string. A good method for repairing a rip or tear in your tent is to sew the hole up first, then use a double patch of waterproof canvas, (one on either side of the sewn up tear, about two inches larger on all four sides than the tear they cover). Sew the inside patch over the tear, and then use fabric cement or glue to patch the outside area, making sure to cover your needle holes completely and sealing the edges fully to prevent seepage and water trickle. You also will want to waterproof the canvas on occasion and keep your tent out of the sunlight whenever possible because the sun can dry the fibers out and encourage cracking.
Lastly, when storing your tent, consider that what makes a great shelter for you and your family will be just as appealing to other creatures, and if given access they will take up residence. So store your home away from home in a place where critters aren’t so common. Consider some mothballs or cedar chips to keep the creepies out and a nice shot of febreeze to hold off the must. When you notice small tears and stains, deal with them then because they never get any smaller on their own. With care and maintenance, your rugged surplus army tent will last you a lifetime and then some. Happy camping.