I know you said you're just a hobbyist, but a positive air pressure respirator is just the way to go when you're a welder. It's basically an over the head setup that works like those old hazmat suits you'd see in movies. It's got a fan or something blowing in slightly higher pressure air, so none of the air outside can get in as the air is always moving out due to the difference in pressure. What it means is that ALL particulates, gasses, and whatever else you don't want to breathe have no way to get to you unless it somehow gets into whatever is supplying your air.

They make fanny pack sized units with batteries that are mobile, which works for me as I've had to go down in ships and up on scissor lifts to do welding before. I believe they also make slightly bulkier stationary setups that are significantly cheaper but often require other equipment (eg. Specialized compressor) to function. The all in one setup i got is quite expensive (about $2k new) but I managed to find it for quite a bit cheaper used on ebay. I'm comfortable doing this as I've been a welder for 15 years and have experience with quite a few different PAPR systems and am confident in my ability to troubleshoot any problems. If you're completely relying on something like this for safety without really knowing how it works, I'd recommend definitely getting something that comes with a warranty. The parts can be finicky, delicate, and expensive.

Okay, look, I admit it: I own a welder, but I'm not a welder. My experience shopping for welding masks is limited to places like Harbor Freight and Amazon.com, and the notion of searching for "welding respirator" or "welding PAPR" instead of "welding mask" never crossed my mind until reading this thread.

Manganese does not split hydrogen from water molecules. I think you might have it mixed up with magnesium, which does but not at a high enough rate to release an explosive amount even if it's liquid water and solid magnesium.

Sodium and lithium do release enough hydrogen to be explosive and enough heat to ignite it when they are solid in liquid water, but I suspect the gases wouldn't do that. The density is way lower, and if it does prefer stripping hydrogen from water molecules to just pairing up with the the O2 ones (some reading suggests that this is the case for solid sodium exposed to air), I bet the recently freed hydrogen will take that heat and just join up with the free O2 itself before it has a chance to build up. And the source would also be more spread out, so there wouldn't be any reason for a high concentration to form even if the hydrogen doesn't react immediately with O2 in the vicinity.

Not that welding isn't an explosive risk, but it's usually due to the tanks used directly by the welding and/or explosive/accelerant materials in the vicinity of the weld.

If someone offers you $80k to weld their fertilizer tank because it's too full and they want to add an extension, politely decline and consider avoiding the general area until after it explodes.