Tar usually reacts to rain by shedding much of the water from the surface. Fresh, thin, damaged, or poorly bonded tar may spot, smear, dull, or let water reach the material underneath.
Rain is one of the reasons tar was valued. A sound tar coating makes water gather and run rather than soak straight into timber, rope, or a porous surface. This reduces wetting and helps slow the cycles that cause swelling, softening, decay, and surface breakdown.
Fresh tar can behave differently from aged tar. If rain hits before a coating has settled, it may mark the surface, leave dull spots, carry loose tar into streaks, or create an uneven finish. Some traditional tar coatings remain slightly mobile for a while, especially in warm weather.
On old tar, rain can reveal weak spots. Water may bead on healthy areas but darken bare wood, sit in cracks, or creep under lifting edges. Repeated rain can also wash dust and loose oxidised material from the surface, making older tar look cleaner in some places and thinner in others.
The main thing rain tests is continuity. Tar performs well when it is bonded, maintained, and not asked to bridge large gaps. It performs poorly when cracks, joints, end grain, and hidden damp have not been dealt with.