Applying a seam sealer to the inside of your tent can help increase its waterproofing. Most seam sealers come in a tube of glue-like gel that you apply directly to the seam—applying sealer to the seams to help moisture coming through. You can spray waterproof coating on the inside of your tent, but this is often unnecessary. Rain will hit the outside of your tent, so focus on applying any waterproof coating to the outer sheet of your tent. Remember to waterproof your tent outdoors. The liquids, sprays, and gels tend to be pretty intense chemicals, so try to do it in a well-ventilated setting. You'll want to leave your tent set up for at least a day or two, so take a look at the forecast to get a couple of days of continuous dry weather. Before applying any waterproof coating products, your tent should be clean and dry. You can clean your tent by rubbing it down with a soapy sponge, making sure you get any dirt and grime off the groundsheet. Next, open the zips on the inner and outer and rinse off the soap in a bucket of clean water. Hang up your tent to dry. Make sure it's completely dry before applying waterproofing. If it's wet, it will reduce the product's effectiveness.