Nondestructive checking hardness of substrate after plating

We are small "tier one" supplier to a variety of manufacturing customers.
One of our customers in the aerospace industry,the part we manufacture for them is required to be heat treated to 40/44RC and subsequently Zinc/Nickel plated.
The problem we have is that the parts are checking OK after heat treatment but after plating we are getting a lower reading through the plating.Our customer insists on carrying out 100% nondestructive testing on these components and is rejecting our parts.
Is it possible to accurately check the substrate hardness under these conditions.

Denis,

My understanding of the subject is that if you want to test a base metal hardness you have to test directly onto the surface. Trying to test through a coating means that you are getting a composite result. As zinc nickel alloy plating is not a hard coating this will have the effect of reducing the observed hardness value, especiall if the test is being carried out with a superficial Rockwell hardness tester.

Ensure that your customer is testing the steel directly and not through the coating. If they are testing through the coating you need to let them know that the observed value they obtain will not reflect the true value of the base.