WD-40 is really not a rust breaker.... (at least not a good penetrative rust breaker) I would try Kriol or PB Blaster (from an automotive store, PB = Penetrative Blaster)
I just had to crack loose two big screws on the broken machine. A couple days of PB Blaster and Kriol and then using the biggest longest screwdriver that would fit, I had my DD hold the base while i used one hand to press the screwdriver as hard as i could towards the screw, while my other hand was using the set of vicegrips that were attached crossways on the screwdriver... Like so...
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