I am constructing a concrete brick retaining wall three ft out from and curving into an existing RR tie wall. About 10ft of the old RR tie wall will not be "covered" with the new wall due to established plantings. The capstones on the new wall are at the same level as the ties and the customer would like to have the capstones continue on top of the RR ties. What is the best material to use for a waterproof mortar base on top of these old ties. The ties are stable but moist and old. prob less than 7 years of life. I was thinking 1/4 inch plywood strips painted with two coats of acrylic. Am I totally off on this? Would it hold the stones?