President Trump has announced plans to assign tariffs of 25% on all steel imports to the U.S. and 10% for aluminum, though full details – including exemptions – aren’t available. A broad-based tariff should prevent “country hopping”, but exemptions could be used as a carrot in ongoing trade negotiations including those with Canada and Mexico (26% and 6% of imported products being investigated in 2017) and South Korea (6%). Exemptions can also be reduce the risk of retaliation against all American metals exports. The most significant exposures are to Canada and Mexico (both $14 billion...