To apply tar with a brush, use a stiff natural or disposable brush, spread a thin coat with the grain, and push the tar into cracks and end grain without leaving heavy pools.
A brush is usually the simplest tool for tar because it gives control. It can work tar into rough wood, around knots, under edges, and along rope or canvas surfaces better than a roller. The brush does not need to be precious; tar is difficult to clean completely and may permanently darken the bristles.
Load the brush modestly and spread the tar in long strokes. On wood, follow the grain and return to end grain, joints, and cracks. On carved or textured objects, brush out from corners so tar does not collect in thick glossy pockets. If a surface looks wet enough to drip, the coat is probably heavier than it needs to be.
Brush work is also useful because it reveals problems early. If the tar beads, refuses to wet the surface, or pulls away from patches, the material may be damp, dirty, sealed, or incompatible. Stop and correct the surface rather than forcing more tar onto it.