Tito:
I am wondering where you got the zinc white board. An oil ground is wonderful to paint on.
That said, I don't do it very often. As Michael said, it is a lot of work, and it is messy and you have to be pretty careful to do it right.
There are a number of commercially available oil priming products available on the market that will work in a similiar fashion. I might recommend you check those out before you dive into coating the board with Flake White.
Or, you can indeed put a coat or coats of Flake White over it, but I would recommend you go very minimal on the linseed oil. After all, you are looking for the benefits of a lead white ground - don't oil it up too much or your ground will become too fat.
I think that if you waited for at least two weeks before beginning your painting, that should be sufficient time for the lead ground to setup sufficiently. Scrub it in thin with multiple coats rather than thick with fewer coats. Dry it and then wet sand it inbetween coats - use wet/dry sandpaper - sand it wet, and then dry the board with disposable towels. The water will keep the dust from getting into the air. Wash your hands, area and tools used very carefully after each sanding. When it is done, let it dry for a day or two and then buff it to an ivory sheen with a soft cloth. Let it cure for two weeks (my opinion - others may say up to 6 months) before beginning your painting.