I don't know if it's the same for an ETF, but for mutual funds:
Permanent U.S. Withholding Tax Relief for Non-U.S. Investors in U.S. Mutual Funds
http://www.klgates.com/permanent-us-withholding-tax-rules-for-non-us-investors-in-rics--a-new-distribution-opportunity-01-12-2016/
I had a 30%...