Dividends paid on U.S. stocks will be subject to withholding taxes for non-residents. A finalized W8 form will allow for a reduced tax rate if a treaty exists between the United States and the holder's country of taxation; however a valid W8 only allows for a reduction in the tax rate not an elimination of the tax completely.
Note, that dividends paid on U.S. ADR shares are generally withheld at source and will not be eligible for a reduced tax rate.
Clients should consult their tax advisor on how to claim a refund or tax credit.