Tar can make canvas more water-resistant, but it usually creates a heavy traditional fabric rather than a clean modern waterproof textile. Use it for covers and rustic craft, not delicate gear.
Canvas treated with tar sheds water better because the fibres are coated with an oily, resinous layer. This can work for outdoor covers, tool wraps, rustic awnings, period props, and storage rolls. It is a poor choice for lightweight clothing, sleeping gear, or anything that must be odour-free and flexible.
The canvas should be dry, heavy enough to tolerate treatment, and tested first. Apply a thin, even coating and work it into the weave without flooding the fabric. Too much tar can make canvas heavy, sticky, crack-prone, and difficult to fold. Uneven application can leave pale weak spots and black greasy patches in the same piece.
Treated canvas needs plenty of airing before use. It may still mark adjacent objects, especially in heat. For a craft project, tar gives atmosphere and water resistance. For technical outdoor equipment, purpose-made waxed or coated fabrics are usually more predictable.