When in doubt, Google it. The technique I learned and like the best is to prime the canvas with Kilz2 Latex Primer. This can be found at most paint, hardware, and home supplies stores. It's commonly used to seal surfaces to keep out water, and to seal in stains to prevent seepage through paint layers. It creates a great surface for alcohol ink painting. Enjoy! Let me know how you like it.
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Thanks to the wonderful alcohol ink artist, Brittainy Bolding, for sharing her secret on this topic in her YouTube video, How to Prep Your Canvas for Alcohol Inks.