Going on the fact that you may have had the tent in storage for two years and that it may have been new, there may just be an easy solution for you. There are various types of canvas materials ranging from traditional cotton canvas to acrylic coated canvas. As such, different remedies may apply.
Acrylic canvas has been treated with fabric protection to resist water and dirt and has a visible sheen and plastic feel to it compared to cotton canvas, which is dull in comparison and softer. The problem you may have encountered is caused by the fact that canvas dehydrates over time, particularly in hot and dry weather. It’s a natural fibre and simply needs to be rehydrated – you may have unknowingly done this by having the tent up in the rain already.
If the material is new, there are all those stitching holes to contend with. It needs to be re-hydrated and ‘fluffed up’ to allow the fibres to swell and seal the material making it watertight once again. If you have not used your tent for a while you should erect it at home and give it a good soaking with a hosepipe before going on holiday. Depending on humidity in dry areas this should be done every six months or so. Of course you could just wait to go on holiday and put up with a leaking tent – it should seal itself off eventually.
There was the theory that by touching the damp canvas you will cause it to leak, in reality touching is just making the tiny droplets combine into a bigger droplets or pool. Once the tent has been soaked it is very important not to pack it away while wet or damp, not under any circumstance, as this is when it becomes prone to attack from what is termed diamond spot mildew, a fungal growth that destroys the natural fibre of canvas. After soaking let the fabric dry to the touch, fold it up and pack away before sunset and dew falls. If you are leaving camp on a rainy morning and have a wet tent you need to get that tent into the sun and dry within 24 hours if possible.
Longer and you are promoting the possibility of mildew growth. If you are breaking camp in the morning and it looks like rain pull the tent down the night before and pack it up, if you can. And if your tent still leaks after a good soaking there are number of products available from caravan dealers that will assist in obtaining improved waterproofing. These range from aerosols to paint on applications