Why is milk or sandalwood paste used when making vastra from cotton?

The cotton vastra represents the Central channel (Sushumna nadi) of the Cosmic energy system (Kundalini) of the body of the embodied soul (worshipper). The seven cotton beads in the vastra are associated with the seven chakra (centres) of the Kundalini system, in the body of the embodied soul. The cotton thread joining these beads is the link of the manifest divine consciousness providing Sattva predominant frequencies to the entire body. When the attributeless divine consciousness in the Zero (Shunya) manifests as per the spiritual emotion of the embodied soul towards God, its colour is white and shining. In contrast the attributeless divine consciousness is like transparent water, but in this state it is dormant. The cotton beads and the thread joining them are the carriers of the manifest divine consciousness in the body. Milk is used to make the constrictions in this thread because the flow of manifest divine consciousness, is like the flow of milk. When this divine consciousness is provided to these beads, these centres (chakra) of the Kundalini, in the form of beads in the body are activated and the embodied soul (worshipper) comes into duality. It then carries out the functions related to the physical body in the universe with the help of non duality.

Use of sandalwood paste is better then milk when making this vastra. Due to the coating of sandalwood paste the frequencies of the deities are activated early and get attracted to the cotton vastra. By adorning the deity’s neck with this Sattva predominant vastra, the deity assumes It’s ‘with attributes’ form faster and begins functioning for the embodied soul (worshipper) in a shorter span of time. (Divine knowledge received through the medium of Mrs. Anjali Gadgil, 22.4.2005, 12.22 p.m.)

Reference:

'What is the importance of the substances used in ritualistic worship?', published by Sanatan Sanstha.