Unfortunately, there is no grade of walnut lumber that can promise 100% sap-free heartwood on both sides of the board. Our suppliers provide walnut that is 80%+ heartwood on one side. Sometimes boards can have more heartwood content, but it's just not guaranteed.
Basically, if you don't like pale sapwood in your walnut you'll need to
- cut if off, or work around it
- consider using walnut veneer, or
- consider a dye and stain technique to hide the sapwood
We posted a more informative blog, which we encourage you to check out. It'll take about 2 minutes of your time and will help you out.