Need advice on priming board

[Tito Champena]

I am the one who is planning to paint a portrait of his granddaughter, using the verdaccio and glazes technique. I have purchased a wonderful piece of hardboard that has been primed with linseed oil and Zinc White. However, because it does not look white enough for my purpose, I'm planning to apply a second layer of priming but using Flake White instead of Zinc White. I'm asking for advice now because I don't want my granddaughter complaining 60 years from now, that her portrait has just peeled off from the hardboard.

This is the way I plan to proceed:

1. I intend to use Flake White in linseed oil because acrylic gesso will not stick to a surface that has been primed with an oil paint.

2. Before applying the Flake White I will spread a thin film of linseed or walnut oil on the surface to improve the adhesion.

3. I will use turpentine, not oil as a medium, in order to comply with "fat over lean" rule.

4. I will not start painting until the Flake White layer is completely dry. (Incidentally, how long will it take to dry?)