Where did you pull 6 months number from? Why don't you learn regular welding first and see how long it takes to master before throwing out random numbers? Different weldings use different techniques, so having a steady hand is the only transferrable skill, everything else will have to be relearnt. Also, if your plan is to hand out security clearances like confetti, then what's the point of security clearances?
Well, would you actually believe I've actually done a university welding course that covered the different techniques?
So I do think 6 months should be more than enough time to learn a new welding technique.
In a wartime scenario, the security clearance process will have to be massively, massively scaled up. There's no alternative to this. But it helps that most welders already work for a company, so are a known quantity. Plus most people in China haven't been abroad before or even know a foreigner.
Steel don't go straight from steel mill on to a ship. There are a lot of steps involved, and those steps are the bottle necks. You can produce as much naval steel as you want, but they would just be sitting in the yard because there aren't enough people and machinery to process them.
Nepoleonic Wars didn't feature aircraft carriers, so it's totally irrelevant. In fact, bringing up anything pre-2000 would be irrelevant.
Modern warships take years to build. Given dry-docks are fixed and known targets, on top of the precision and range of modern weapons, completing modern naval vessels in a war time situation before they get destroyed would be nearly impossible. In modern naval warfare, you are fighting with what you've got.
As I've said, it is a long process to build up such a Navy.
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3 years ago, Patchwork enlightened us that the US military was highly unlikely to generate enough fires to penetrate the IADS on mainland China. And since then, the military balance has swung sharply in China's favour, even though the US is trying to pivot.
And recent events have demonstrated that China likely has a chokehold on US military production. Before Trump caved in, look at how Raytheon was just 3 weeks from stopping missile production, as per Gavekal Research.
In comparison, we could expect China to massively ramp up munitions production.
So then you end up with a stalemate, where:
1. China is dominant in the Western Pacific, but can't reach the USA
2. The US is dominant past the Western Pacific, but can't reach China
In such a scenario, shouldn't China aim to build a Navy that can win?
Sure, they might sue for peace. But given that the situation is likely unresolved (like in the Napoleonic Wars), both sides would see a resumption of the war as likely, and build up their navies during this "cold peace"