Specialised wire for windscreen wipers: yes. Steel plates for repairing broken bones: no. Bill Brebrick, sales manager for Zapp Precision Wire, has been learning fast about what the US government will exempt from its tariffs on imports of steel and aluminium.

Specialised wire made in Germany, sold by Zapp to be made into windscreen wipers in Mexico, is available only on import, the US department of commerce has ruled, and has therefore won an exemption. Shaped plates used to repair badly broken bones, however, could in the government’s view be made in the US, so Zapp faces tariffs on those unless it finds a domestic supplier.

The Trump administration in March introduced tariffs of 25 per cent on steel and 10 per cent on aluminium, and also set up a process for US importers to request exclusions. As application for requests on tens of thousands of products have flooded in, many companies said they were finding the bureaucracy frustrating and the decisions bewildering.

Winners and losers are being chosen seemingly arbitrarily by the government, they complain, distorting markets and disrupting supply chains. Meanwhile, a backlog of decisions continues to grow.

“Manufacturing was doing so well,” said Mr Brebrick. “People would like to get back to business as usual. Let us compete on a level playing field and leave us alone. No more regulation, please!”

The sour joke among opponents of Mr Trump’s strategy is that while his tariffs may do little to bring manufacturing jobs back to the US, he has at least been highly successful in creating work for the federal government.