Do NOT use silicone spray.It won't be waterPROOF, just water repellant.
1.Set up tent very taut on warm summer day or in a warm room.
2..PURCHASE: 1 qt. ODORLESS Mineral Spirits, 1 tube GE clear silicone caulk, small 6" paint roller and pan, roll of blue heavy duty "shop towels".
3. PROCEDURE: ->mix 5 parts odorless mineral spirits and one part GE clear silicone caulk in a pint sealed jar and SHAKE FOR 2 MINUTES ->pour part of mix into paint tray and roll on to a tautly set up tent in 1or 2 ft. square areas at a time ->wipe down immediately with blue shop towel. This leaves a light but waterPROOF coating.
Let fully dry for 48 hours.
Repeat the process on the floor exterior with a small table inside the tent moving the floor over the table top as needed for a flat surface to permit using the roller.
I've done this on two tents and it has lasted for many years.They were silnylon tents so if your cheap tent is urethane coated inside be sure to do this procedure only on the outside.
My advice: go to WalMart and buy their best backpacking tent. The now have decent quality tents.
Eric B.