I...uh....wait...ummm...hold on....wait...

DM: Scribbles a note "Without the rust it seems like a serviceable crown, but not too fancy."
Note to lost heir: "You see the crown and you think as it... looks at you. This should be your crown. You wants it. They shouldn't keep it from you. Steals it, hides it, it came here for you".
DM: "Probably worth some gold."
- The player: GUYS! I'M A KING NOW!!! BOW TO MEEEEEEEE!!!

"you feel the spell take hold, but for some reason the crown remains rusty"
Then you pivot that the rust is a powerful illusion or some kind of curse cast on the crown by someone related to that backstory to keep it hidden. Then while your players try to figure out why simply cleaning the rust didn't work, you try to figure out how to weave in that backstory sooner than later.
If you're really not ready for it to happen, make sure they have some other quest to do that has a pressing time limit.
- you're definitely right about the time limit. at that point you are about 5 minutes away from every spell in the party's arsenal being cast on that crown, followed by the main quest getting derailed by the mystery of the plot armored artifact.
  
  I'm extremely naive when it comes to tabletop RPGs
  Is there any kind of "plot says no" response to magic? Something like the doors in oblivion where you need a key to unlock
- If you’re really not ready for it to happen, make sure they have some other quest to do that has a pressing time limit.
  So, distract the players as if they are children. Good idea.

My GM solution: the rust is actually blood, and the crown needs a fresh coating to activate its narrative. If need be, make it belong to a certain bloodline, such as royalty.

The crown completely disintegrates, as it was rust all the way through
Sorry, Mario, the real crown is in another dungeon.
- the real crown was the XP we collected along the way
  
  The real mario is the rust we collected along the XP

It’s artifact level - a cantrip simply doesn’t work on it. When the players ask why, you just tell them they don’t know - neither does anyone in the town/city whatever they’re in.

Having your complex plot get fast forwarded because of a cantrip, priceless 😆😆😆😆😆
- I once fast-forwarded a complex plot through a GM-sanctioned bit of fluff.
  The party had been invited by their uncle who turned out to be recently murdered when they arrived. Of course they investigated. At one point I had my character wrote a letter to the rest of the family to inform them of what was going on. I actually produced the letter as a handout. Since I had no idea about the date I asked the GM and he told me to pick anything in summer.
  The GM s happy with the handout and it was deemed canonical.
  A few sessions later he noticed that I had picked something ahead the end of the summer and the bad guys' plot was about to kick off at a specific date right after summer ends. So suddenly the adventure went from "careful slow-burn investigation" to "mad rush to the location of the finale".
  Oops.
  
  Couldn't they have gone the other route and made the villain's plans a year later? But sounds like it was a lot of fun the way it was run!

What metal is the crown made of?
The spell only works on iron and iron-heavy alloys. An advanced version of the spell exists but the players don't have it yet.
- Technically, rust can only occur on iron-heavy metals and alloys. Otherwise it's just called oxidation.
  The difference with "rust" is that rust will eat into the metal and change its shape, while oxydation only changes the surface color and texture.
  Edit: yeah... Rust is a specific type of oxydation, it wasn't really clear from my comment. What I wanted to say is that rust implies the material is iron-based!
  
  Well, not quite. Rust eats into iron because oxidised iron is larger and much more brittle than unoxidised iron, physically ripping itself out of place.
  Many oxides arent that much larger than their base metals and form a nice patina protecting the metal underneath, like in aluminium.
  Other oxides destroy the structural integrity of the metal and eat into it, forming corrosion. Rust is just corrosion specific to iron.
  
  This is not true. Oxidation is a broad type of chemical reaction involving the loss of electrons. Rust is a type of oxidation, much as a square is a type of rectangle. Oxidation can occur on the surface level (tarnishing of some metals, passivation of aluminum) or throughout (combustion). Rust actually only occurs on the surface as well, but the iron oxide is less dense than the metal and it increases the available area of the surface exposed to oxygen.

Easy. He needs to roll 100 on a single d20 or the spell fails and creates a big neon sign above the player characters head that follows them everywhere and reads "annoying little shit"

Thats DnD, though. You're not the narrator, you're the benevolent god allowing the story to unfold.
I played recently with a newer DM who had written this complex story and kept trying to weave in obvious set pieces for us. At first, I played along, but when we started to go off track, he introduced an omnipotent NPC to help keep us on his path. I was done at that point. I'm not here to listen to a story.
If I find a clue early, I understand it might not make sense until later.
- Going by your entire comment alone.
  I have juuust the right youtube video for you: https://www.youtube.com/watch?v=EkXMxiAGUWg

The rust is removed, but there's significant chunks missing due to the rust settling in. It is still unrecognisable and needs restoration.
Or something magical based on what the artifact does

I...uh....wait...ummm...hold on....wait...

the rust scales begin to fall and as the entire party squints to see the results, ROLL FOR INITIATIVE AT DISADVANTAGE (fuck a few dragons will get me out of this shit)

improv intensifies
- I learned that best things come from the right balance between preparation and improvisation. And that balance is approximately 20-80 respectively, at best. I figured that as a DM, I'm also playing, so I roll with my fellow table partners, as the story is unexpected for me as is for them.
  
  Yeah. At this point I try to prepare scenes rather than plots, so hopefully I'll be able to use my painstakingly prepared battlemap later, rather than not at all.
  But it's fun when the players throw a total curveball, and I need to come up with something on the spot.

Twist: You think this is the legendary lost crown of Foo? Some rotten trash you grabbed in a dungeon just happens to be the thing you've been looking for all this time? Pull the other one! It's been so ravaged by time that none of the markings or engravings are clearly visible. Best you can hope for is that some merchant will buy it off you for scrap.
Even if the PCs think this is the lost crown of Foo, only the kingdom's last grandmaster artificer can conduct a conclusive test. Assuming you even find them, it's not like they take appointments from any dirty old adventurers off the street.

How tf do I pronounce that*
- I don't know if you're joking, but just for fun: Press-T-digi-tay-shun

If you've railroaded your campaign that much you're a bad GM. It's not your story, it's your players story.
- I hate this take a lot, I'm gonna be honest. I don't care if his game is so on rails that it's set on the fucking orient express. As long as the players are having fun with the game, and the GM is having fun with the game.. that's a good GM.
  
  So a player that told you from beginning what he wants to do, which doesnt fit into your story, should they be forbiden to participate?
- Rollercoaster are fun yet have rails.
  Are you even a GM to allow yourself such snap judgment? But for you know, we GM/DMs are not your employees RPGs are a group collaboration.
- How is this in any way railroading?
  
  The DM determined that A) the players would find this crown, B) they would not clean it when they found it, and C) it would get cleaned at some point the DM decides later, whether the players wanted it to or not. Good for a book, bad for D&D.