I have recently had to tackle some rusty nuts/bolts that looked like they hadn’t moved for years. Sprayed some RP90 on them, left for a few minutes and they come off without resorting to an angle grinder.
Readily available from your local Toolstation. Vastly superior to WD40.
Not surprized - WD40 is not a penetrating fluid.
It is Water Dispersant - formula 40
I use plus gas formula a, works very well