troughout my nvq i was taught to use a conditioner/treatment after, but when i was doing my graduates course at wella academy they said you should always use a closing down treatment like wella service or blondor seal and shine before the toner because it stops the bleach from developing, evens the porosity and balances the p.h. level of the hair so the toner will take more evenly
ive also pre-lightened someones hair shampooed and conditioned(to help detangle) cut and then applied toners to different sections of hair and it has developed fine, but id defo recommend to rinse thoroughly to ensure it is completely out