Trump also has repeatedly summoned the head of the U.S. Army Corps of Engineers, Lt. Gen. Todd Semonite.

The barrier that President Donald Trump wants to build along the Mexico border will be a steel bollard fence, not a concrete wall as he long promised, and the president is fine with that. He has a few other things he would like to change, though.

The bollards or "slats," as he prefers to call them, should be painted "flat black," a dark hue that would absorb heat in the summer, making the metal too hot for climbers to scale, Trump has recently told White House aides, Homeland Security officials and military engineers.

And the tips of the bollards should be pointed, not round, the president insists, describing in graphic terms the potential injuries that border-crossers might receive. Trump has said the wall's current blueprints include too many gates - placed at periodic intervals to allow vehicles and people through - and he wants the openings to be smaller.

At a moment when the White House is diverting billions of dollars in military funds to fast-track construction, the president is micromanaging the project down to the smallest design details. But Trump's frequently shifting instructions and suggestions have left engineers and aides confused, according to current and former administration officials.

Trump has demanded Department of Homeland Security officials come to the White House on short notice to discuss wall construction and on several occasions former secretary Kirstjen Nielsen in the early morning, waking her up to discuss the project, officials said.

Border barrier prototypes stand in the San Diego area in April 2018. President Donald Trump has been intensely interested in the design and appearance of the project.

Trump also has repeatedly summoned the head of the U.S. Army Corps of Engineers, Lt. Gen. Todd Semonite, to impart his views on the barrier's properties, demanding that the structure be physically imposing but also aesthetically pleasing.

"He thinks it's ugly," said one administration official familiar with Trump's opinions, who, like others, spoke on the condition of anonymity to avoid being fired.

The president sees himself as "a builder," said David Lapan, a former Homeland Security official who worked at the department when it spent more than $3 million on the construction of eight border barrier prototypes near San Diego.

"But building high-rises in New York City is not the same as putting up a barrier at the border," said Lapan, now at the Bipartisan Policy Center in Washington. "You're not looking for aesthetics, you're looking for functionality."

Homeland Security officials had settled on the steel bollard design many years earlier, but Lapan said many of the prototypes were built using concrete to suit the president's tastes - and demonstrated that the material was impractical and vulnerable to breaches.

White House officials did not respond to questions about the president's design requests for the barriers and about his conversations with engineers and border officials. Homeland Security officials declined to comment. An Army Corps spokesperson confirmed that Semonite has met with several times with Trump, but referred inquiries to the White House.

The president - who repeatedly promised to force Mexico to pay for his desired border wall - has pledged to build 400 miles of new barriers by next year, a goal he reiterated during an immigration speech at the White House Thursday afternoon. The plan would likely require him to reprogram additional taxpayer funds from military budgets.

Trump's changing tastes are potentially driving up the price. He remains adamant that the barrier should be painted black, despite warnings that it would significantly increase construction costs and maintenance budgets.

"Once you paint it, you always have to paint it," said another administration official.

Trump also has changed his mind repeatedly about the structure's height, urging engineers to make it as tall as possible, though his desires have been tempered by cost concerns and engineers' worries about structural integrity.

The president's critics are determined to stop or slow down a project they denounce as a wasteful monument to the president's vanity - more symbolic than security-minded along a border that cannot be completely walled off because of rugged geography and the Rio Grande.

In the compromise deal to end the government shutdown in February, Democrats agreed to provide $1.4 billion for the border barriers, far less than the $5 billion Trump sought. Democrats also inserted language limiting the expenditure to "operationally effective designs" that U.S. Customs and Border Protection already uses.

With the White House using an emergency declaration to get an additional $2.5 billion in funds diverted from military budgets, Trump will face no such congressional scrutiny, potentially giving him more latitude to tailor the structure.

Before her removal from DHS last month, Nielsen had "very specific meetings" on the wall project, another administration official said. She thought the president's acute interest in the barrier's appearance became a distraction from other, more pressing border issues, the official said. Nielsen did not respond to a request for comment.

On the 2016 campaign trail, the president told crowds of chanting supporters he wanted the wall to be big and "beautiful," and those two qualities continue to drive his requests.

"He thinks not only can the wall be effective, it doesn't have to be an eyesore," a DHS official said. "He wants one standard uniform height. That's what he's going for, and we have to match that with operational reality."

The steel bollards remain vulnerable to sawing, but Homeland Security contractors have filled the hollow cavity of the metal with an undisclosed compound to make them more difficult to cut. The material is poured to a certain height - which officials declined to specify, citing security reasons - and ordinary concrete, which is cheaper, is added after that.

One official called the saw-resistant compound a "secret sauce" in the bollards, declining to provide more detail.

Trump often brought up the construction of the barrier at unrelated meetings, and aides learned to bring prep books - and even sketches - to address his questions. He often grew frustrated when he would learn that more of the barrier was not built, the current and former officials said.

He continued to insist on speeding up construction, blanching at suggestions from aides that it would take many years, according to former administration officials. Trump frequently delved into the minutiae of contracts and suggested that some of his friends in New York would have ideas on how to build it faster, officials said.

Trump has been so eager to see sections of the barrier in place that he has taken credit for fencing construction that was planned well before he took office. Along one 2.25-mile span of replacement barrier in California completed last year, a plaque welded to the structure heralds it as "the first section of President Trump's border wall," even though U.S. agents say planning began in 2009.

At periodic meetings to update the president on construction progress, sometimes held more than once a month, Trump has asked questions about how border-crossers might be able to "cut a hole in it, dig under it, climb over it," in the words of a meeting attendee.

Some of the president's requests have led to significant alterations in the design. In particular, he insisted on boosting the height of the structure to 30 feet, far taller than the 15-to-18-foot range that CBP officials had previously settled on as optimal.

The advantages of the 30-foot design were made apparent after the administration paid for a the series prototypes, another former official said.

"We were able to test what happens when you put someone up that high. They freeze up," the former official said. "There was significant deterrence value to putting people on a 30-foot wall."

Said another official familiar with the president's desires: "He always wanted to go higher."

Barbara Res, a former Trump Organization executive, said Trump would get closely involved in design aspects of his hotels, resorts and other development projects. "If it was very visible, he got very involved," she said.

Res said the president cared deeply, for example, about the kind of marble and brass in Trump Tower, as well as the kinds of floors in the apartments and the kitchen finishes - details that Res characterized as "trivial."

But for a massive Homeland Security project, she said: "Who cares what color a wall is?"

One design Trump panned, according to a former official, was topped by a rounded, barrel-like metal cylinder to prevent climbing. Approved barrier designs include a flat-panel anti-climbing surface that has been field-tested, but the president doesn't like the way it looks either, arguing that sharp spikes would appear more intimidating.

Trump told one group of aides that the metal points would cut the hands of climbers and function as a more effective deterrent.