What was the silver "valhalla spray" in Fury Road?

They are not huffing paint. It has nothing to do with "huffing." They are spray painting their "grills" (teeth) so they will be (as they say in the film) "shiny and chrome" when they go to their Valhalla. Remember, in their Valhalla, they ride forever. It's a kind of statement...it has nothing to do with any toxic or intoxicant effect.

Movies.com interviewed director George Miller, where he revealed his inspiration:

I saw a documentary where young [Cambodian] soldiers would go into war, they had little jaded deities -- and before they ran into battle, they put them in their mouths and just held them with little straps.

Apparently:

This was their ritual before battle -- that, like the Buddha, the chrome paint will help lift them to a higher place. It'll help bring them to Valhalla.

And from an interview with actor Hugh Keays-Byrne, who plays Immortan Joe:

What exactly is the silver stuff you spray on Nicholas Hoult’s face?

That’s like a very euphoric drug.

As George Miller explains further in the interview:

[Immortan] Joe was a member of the military who used his leadership skills to build a cult.

"He was a colonel named Joe Moore. He was military, and he organized everybody, eventually being able to exploit a cult as he took over this dominance hierarchy."

Speculation:

• My guess is that cult leader Immortan Joe came up with the "Valhalla" explanation to manipulate his followers into drugging themselves. The War Boys don't actually know that the spray contains a drug. They genuinely believe it will help them get to Valhalla.

Ok so from what I gathered, they're effectively 'huffing paint', in order to make their suicide easier to go through with. It's narcotic effects make them more willing and geared up to 'enter Valhalla'. Notice how both Nux and the other War Boy both widen their eyes and smile maniacally after they've been sprayed. The visual element is likely a reference to their obsession with cars, so they're 'chromed' up, but I took it to be more for it's effects than it's look.

I think maybe people are looking too much into the spray.

Mad Max IS visual storytelling at its best. The spray could be their ritual before they commit suicide (enter Valhalla) and perhaps they use it to mark their bodies so they can be identified when they enter their next life. But I also like the idea that it gives them a quick high (i imagine inhaling any kind of solvent would make you go a bit dizzy) so their suicide is slightly easier on them.

Writing this I had another thought: They say 'Witness me' before they kill themselves... maybe the spray is a visual indication to everyone too far away to hear that they are about to enter Valhalla?

But until we get a quote from the Director it's all just interpretation and as a community... I'm sure someone will hit the nail on the head and give everyone a satisfying answer.

Just as everyone has said, but I'm assuming you're more confused at why they did it. All those people who are painted white and spray their mouths (I forgot what they're called) are bred at a young age to believe everything Immortan Joe told them. He basically created an army that would do anything for him. Everything they believe in is because he's told them that, they risk their lives because they think Valhalla is real. Some people hated it because of that aspect, I personally think they just didn't understand it. It's like how Kim Jong-Un makes North Korea believe he's a gold medalist, doesn't poo etc. it's a pretty great way to create an army, they're bread to defend and attack for you. The paint is just a metaphor for what they have been told they'll become and they do it before they die in an act to help their master (Immortan Joe). Shiny and Chrome

Its simply to "pretty up" for their acceptance in Valhalla. It has nothing to do with a drug, suicide or the like. The one character also refers to shiny things a few tines in the movie especially when seeing the wives at the truck. Its simply more of a tribute to Valhalla and its pretty. Think of it as their "gold" as far as being pretty, valued, of high significance. Not a drug or related whatsoever

They are huffing. They are out of the tree. It is the equivalent of soldiers in WW1 or WW2 having a shot of rum before they went into battle to face there death.