We don't do that here. The GM provides the model of physics the players accept and expect. If the GM just says "nah" when stuff is inconvenient, players don't know what to expect, and the world becomes inconsistent.

A big part of the GM's fun in TTRPGs is improving off that. Players always ruin my plans, but that's part of the game.

Really, what the DM says goes. So if you want to be boring you can just say it doesn't work for some reason. The answer above re: pivoting to it being a powerful illusion spell or something so there is a reason the spell didn't work is a lot more compelling and interesting imo

Technically there could be. After all, the GM has final say. But players will want to search for a reason, because they expect consistency. Spells don’t typically fail without reason. That reason can be a low die roll if the spell description calls for it, but many spells (like Prestidigitation) don’t require a roll.

So having the spell fail “because the plot says no” is inconsistent. It would immediately throw up a giant red flag in the players’ minds, and make them think the item is much more important than they initially realized. After all, if the plot says the spell doesn’t work, then that means something in the world is preventing it from working.

It makes more sense to have the item be cursed, or haunted, or protected by a god, or any other number of things that would give the players some sort of explanation to latch onto. If you keep it vague, the players will inevitably spend a lot of time trying to figure out why it can’t be cleaned. Because they expect consistency, and will keep throwing things at it until they find a reason. So it’s better to just give them a reason (even if you just came up with it in a panic) because that at least gives them some resolution, and they can file it away in their quest list for later.

there's two answers to this question, one is mechanical and one is social. you as the DM can tell the players no not now, and they can't do anything about it, but that doesn't mean they won't try to do something about it, which depending on the group could be an issue.

so in this scenario a good DM could whip up some misdirection, for example set up a traveling artificer who just passed through town a couple weeks back and who the players could track down as a lead - conveniently in the direction of the main quest objective.

this is hard to do on the spot.

"You can certainly try"

The idea was to have some kind of urgency but only once the players were far enough to understand the basics of what was going on. To that end, the date was supposed to be vague so that the GM was free to say "you figured out that the ritual will happen right after summer ends – which is in less than a week".

Then he forgot that the timeframe was vague when I wrote the letter and told me to pick a date.

Unfortunately, this cut out a side plot where our party would've hired another party to hunt down some artifact. That artifact retroactively got downgraded to a red herring for time reasons.

On the other hand, we got an absolutely precious scene where the one party member who wasn't magic-affine and didn't want to be involved with any supernatural stuff had to ride an unnaturally fast six-legged half-demon horse in order to catch up with the bad guys.

Also, it cut down on all the "three wizards and a vintner have breakfast and discuss the state of the investigation" episodes. We had a lot of those.

I didn't know of other type of oxides that eat into the metal like rust does...

But it's true that a "rusted crown" implies that it is iron-based, so the cantrip should work!