No matter how well a tent is constructed or the claims made by the manufacturer, tents leak. Most modern tents are double-seamed and a coating of seam sealer is applied to the seams to keep rain from entering the stitching. During use, these coatings can deteriorate and allow water to seep in. Yearly maintenance of these seams with a new coating of seam sealer or the application of seam tape allows the tent to retain its weatherproofing.
Erect the tent in an open area with access to good ventilation. Unzip the windows and doors to allow for air flow and set any rain-fly aside for now.
Examine the tent seams on the inside and out. Look for any obvious flaws, thread pulls or other issues that may lead to leaks. Apply a heavy coating of seam sealer compound on damaged seams according to the instructions on the product tube. Go inside the tent and apply a lighter coating over all seams in the walls, and around the edge of the flooring, doors and windows. Make sure the sealer contacts all seam threads, and reapply as needed.
Apply new seam sealer to peeling factory-installed seam tape and press it down against the tent seams for 30 seconds to establish a strong bond. Use enough of the adhesive so that it squeezes out of the sides of the tape slightly to get a bonded seal with the seam. Replace old seam tape with new if it's absent or severely damaged. Press new tape down onto seams so it completely covers all threads.
Lay the rain-fly out on a flat surface and check the seams for flaws. Apply a coating of seam sealer to all seams on the fly. Allow the sealant or taped seams to dry for 24 hours.
Remove the poles and lower the tent. Fold all poles and the tent so it's ready for secure storage.
Apply unscented talcum powder to seams after spreading the sealer compound to keep the sealer from sticking to the folded tent during storage.
Use a new package of seam sealer for best results. Old compounds may not have the same bonding strength as newer.