The Hare Kṛṣṇa mantra does not require śodhana, or purification of the mantra:

As enjoined in the Hari-bhakti-vilāsa (1.215, 219, 220):

na cātra śātravā doṣā
narṇasvādi-vicāraṇā
ṛkṣarāśi-vicāro vā
na kartavyo manau priye
nātra cintyo ‘ri-śuddhyādir
nāri-mitrādi-lakṣaṇam
siddha-sādhya-susiddhāri
rūpā-nātra vicāraṇā

There is śodhana, or purification of the mantra, but there is no such consideration for the Kṛṣṇa mantra. Balitvāt kṛṣṇa-mantrāṇāṁ saṁskārāpekṣaṇaṁ na hi. “The Kṛṣṇa mantra is so strong that there is no question of śodhana.” (1.235)

– Śrī Caitanya-caritāmṛta Madhya-līlā 24.331