The 3M tape they use is certainly tenacious stuff, stubborn to say the least.
Was your spoiler full of water? Mine was, a good couple of pints sloshing around in there, only place it would come out was the screw holes, I'm at a loss how so much got in there, it was a genuine spoiler too.

Hi, Yes, very sticky. I haven't cleaned up the spoiler where the 3M stuff was stuck. I will leave that for another day or sell it and the new owner can do it. The car took me about an hour and a half to clean up.

I almost decided it wasn't worth the bother.

No water at all. I did notice there was a small rubber cap on the underside of the spoiler, I don't know if this would be where you could drain it?

What would you use to renew the sticky pads with? Anything specific?

3M VHB, (very high bond), rolls on Fleabay are cheap enough.