I would probably just clean the fans carefully with a brush, make sure they're not jammed solid, blow the dust off the board, then plug it into an old computer. Further tests would show if repasting was necessary. Of course if Bryan wants to resell the boards, I applaud him for taking the time and trouble to clean them thoroughly.
"Bryan used a little vinegar to remove the rust from the heatsink."
I'm intrigued. According to Wiki:
Rust is an iron oxide, a usually reddish-brown oxide formed by the reaction of iron and oxygen in the catalytic presence of water or air moisture.
Was part of the GPU heatsink made from iron?
Most of the heatsinks I've seen are made from Aluminium and Copper, not Iron.
In the case of Copper, the greenish coloured oxide is called Verdigris. When Aluminium oxidizes, it turns white.
Rust is brown, not green or white.
As for using vinegar to remove rust, it's easily available, but I'd reach for my bottle of dilute phosphoric acid, which I use a a flux when soldering some metals. You occasionally get "Flash Rust" forming after using vinegar and phosphoric acid can be used as a rust converter/inhibitor.
Applying a rust converter or inhibitor like phosphoric acid on the rusted surface also helps in preventing flash rusting. It creates a passive layer on the surface that protects it from rust formation.
https://www.toolsadvisor.org/how-to-stop-flash-rust-after-vinegar/
FAQs
What causes flash rust after using vinegar as a rust remover?
Vinegar contains acetic acid that can breakdown the protective layer on metal surfaces, leaving them vulnerable to rusting. This can lead to flash rusting if left untreated.
How can you prevent flash rust after using vinegar?
After using vinegar to remove rust, it’s best to neutralize the surface by washing it with baking soda and water. Then, dry the surface thoroughly and apply a rust converter or inhibitor to prevent flash rust.
Can you use vinegar to remove rust from all types of metals?
While vinegar can be effective at removing rust from iron or steel, it may not work as well on rust stains from other types of metals like copper or aluminum.
What is flash rust?
Flash rust is the rapid formation of rust on metal surfaces shortly after they have been cleaned or exposed to water. This can occur when the metal is not adequately protected or treated after cleaning.
How long does it take for flash rust to occur?
Flash rust can occur within minutes to hours after a metal surface has been cleaned or exposed to water, especially in humid or wet conditions.