Canvas tarpaulins are a high grade sheet and therefore will require a certain level care over its life. These products are usually waxed in order to waterproof it and keep it from deteriorating and therefore after a period of time it will need to be refreshed.
If you have experienced a period of inclement weather or you have had the sheet for a year or more your tarpaulin may need to be treated. This is an easy process and can be done at home, provided you have the space.
Please make sure that the tarp is good condition. Once you have done that the first step is to clean it. This can be done with a fresh cloth and some washing up liquid. Be careful because only a small amount of liquid is needed. Remove as much debris and as many marks as possible. Make sure the cotton is dry before taking the next step.
You need to purchase a waterproofing product such as a fluoropolymer wax or spray, this is the most common. Please refer to the instructions on the bottle/can as each one tends to be slightly different. Usually two layers are applied, the first in a horizontal fashion and the second vertically. For obvious reasons doing this in dry weather is a must. Diluting the wax you are using will reduce how effective it is in the future.
The process is very simple and easy to do. Your tarp should be re-waxed every 1-2 years. When storing your cover it is important it is dry before you fold it; water can cause mould to form in storage. Some Silica gel is a good way to keep your tarp dry when it has been put away.