It is time to make the final payment to my PB and I don't know what to do. He is telling me that the black markings on the white plaster are trowel marks that will disappear with time. (It has been almost 3 months.) They cover nearly all the shallow end and are on the walls in the deep end. They appear black on the white plaster and make long arched sweeps. He also said that because plaster is made from marble, color variations are normal. However, this is very significant--not just here and there. Does anyone have any advice? Thanks!!