I've found something almost exactly identical to the quote you're looking for, but it's much newer than the US Founding Fathers; it comes from Aldous Huxley's Brave New World, Chapter II:
Not so much like drops of water, though water, it is true, can wear holes in the hardest granite; rather, drops of liquid sealing-wax, drops that adhere, incrust, incorporate themselves with what they fall on, till finally the rock is all one scarlet blob.
In context, this quote has a very dark meaning: hypnopædia (sleep-teaching) is being used to indoctrinate young children with the "correct" beliefs and norms of the society, such as that people from different castes shouldn't mix together. The very next paragraph emphasises the meaning of the metaphor about water and wax:
'Till at last the child's mind is these suggestions, and the sum of the suggestions is the child's mind. And not the child's mind only. The adult's mind too--all his life long. The mind that judges and desires and decides--made up of these suggestions. But all these suggestions are our suggestions!' The Director almost shouted in his triumph. 'Suggestions from the State.' He banged the nearest table.
It's possible that Huxley took this idea from some writings of the Founding Fathers, and made a dark twisted version of it for his Brave New World, but I couldn't find any earlier quote or reference to this exact idea (water versus wax dripping on a rock).