Rust-Proofing Re-application on Rusted areas?

Hi guys,

My wife has a Toyota Corolla 2000 model that had been originally rustproofed (Body Zone ata) at the casa, I noticed it is starting to develop rust flakes at the front underside. I am wondering what I need to do to remove the existing rusts and if it's alright to have rustproofing re-applied again kasi baka nag-fade na yung original rustproofing and undercoating nya. How much kaya aabutin and ano kaya puwede ko gawin sa rusted areas nya para di na kumalat, or san ko sha pwede dalin para magawan ng solusyon :(

Sayang naman kasi, wala naman ibang problems kundi yan lang.

Thanks in advance for your inputs :happy: