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Paint patina I can't help you with. Rust now that I think I can help you with if you are patient and stick with it.

On my 29 AA I found a left rear replacement fender that was new old stock. Looked like a brand new fender when I stripped the paint off of it. It took me dang near a year to get it to rust up to match the rest of my truck. To date not one person has yet to notice that the fender was not on the truck for the past 85 years.

On your new welds you will need to give them a bit of texture after you grind / sand them smooth. Try using a sharpened metal punch and hammer to gently put some dimples and pits in. Be creative, maybe some really course sand paper laid on face down on top of the weld and then stuck with a hammer. You get the idea. Once you get a little bit of texture on the metal it will help the rust take hold.
On my fender I started off with the large course granulated Kosher salt. I got the metal wet and then sprinkled the salt on letting it stick. The idea wasn't to totally cover it in a layer of salt but to allow the large granules to stay in one spot for a long time to get a bite into the surface. Then I mixed a salt brine mixture in a quart spray bottle and would mist the fender once or twice a day or when ever I happened to walk by it. I did this for a few weeks. After that I took the fender and buried it in my kids sand box and kept the sand moist for an additional few weeks. By this time I had a pretty nice texture to the metal and all I needed to do was rust it to the degree I wanted. I kept the fender sitting outside my shop door and pretty much every time I went by it I would fill up a large cup of water out of the water spigot by the door and give it a splash of water. Once in a while I would mist it with the saltwater brine. The key to getting something to rust in not to keep it wet all the time but to get it wet and then to let it dry. Metal needs oxygen to rust.
You can try the vinegar, beach, hydrogen peroxide treatments. Just do a google search. They do rust metal quickly but only a thing surface layer that tends to be a rather sickly orange / yellow color.
Best of luck on your project.