It takes a lot of hard work usually. If it’s paint your wanting to remove your best bet is a good paint stripper first of all. Don’t go near Nitromors because it’s now had all the good stuff taken out of it and rendered itself worse than useless. The best stripper I use is exceptionally good (starchem synstryp) but there’s so many different kinds of paint now and again you’ll find one that’s difficult to shift. (Wear gloves because it burns!) After you have removed the paint (several coats of stripper) the world is your oyster. There’s hundreds of different cleaners to use on the raw brass from pure lemon juice, cola,tomato sauce, toothpaste to all the usual proprietary cleaners like brasso or bar keepers friend. It depends on the finish you want, (e.g bright and shiny or dull and reserved). I’ve cleaned and stripped several hundred plaques and you’ll always find one that’s a bummer to do! If your using brasso every week to polish the same plaque it will undoubtedly eventually wear down the brass but as a one off solution to get it shiny I wouldn’t be scared to use it. As for the fire damaged ones they can be a nightmare to clean up. Most times for them I’ve ended up using a cream cleaner and then brasso. The best thing I ever done was to buy a polishing machine. (Bench grinder with polishing mops on!). Always resist from using wire wool or wire brushes as the can cause a lot of damage. If you want to go the gentle way use Huws methods after stripping the paint off. I’m pretty impatient so I go all out to get them done as quick as possible! But you’ll always come across one that won’t go as planned. Best of luck and stick some photos on so we can see the end result. Enjoy!