John Gill Commentary Psalms 86:3

"Be merciful unto me, O Lord; For unto thee do I cry all the day long." — Psalms 86:3 (ASV)

Be merciful to me, O Lord
In my distressed and miserable condition, being an object of mercy, pity, and compassion; this petition is used by Christ in (Psalms 41:10)

for I cry to you daily ;
or "all the day"; every day, and several times in a day, (Psalms 55:17) (Psalms 55:17) constant and importunate prayer is the duty of saints, and available with God, (1 Thessalonians 5:17) (Luke 18:1–7) . Christ was much in the exercise of it, (Luke 6:12) .