A canvas or wood panel must be sealed before priming to avoid Support Induced Discoloration (SID), a condition resulting from impurities drawing up through an unsealed substrate as paint dries, causing the paint to discolor.
Use Golden’s GAC 100 or Natural Pigment’s Rublev Fluid Medium. Pour into a tray and using a brush, apply pressure scrubbing it into the weave of the canvas. Apply two coats allowing to dry between coats. If you are sizing a wood panel, you could use a brush or a wide spatula or putty knife, apply two coats allowing to dry between coats.
Once your have sealed your canvas/panel, you can then apply the primer.
If you are priming with an Alkyd primer on a wood panel, sizing is not necessary.