Ok in case anyone else has a similar problem, I found a great site
http://cryptomining-blog.com/tag/windforce-repair-fans/ that describes how to dismantle and clean these fans. I got the fan out and managed to totally dissemble it without breaking it. Cleaned it up with WD40 (far from ideal but the only stuff I had on hand), reassembled it and the fan spins like new now!
All the other fans are working but gummed up with dust and don't spin freely so I have another 23 fans to fix. While I'm at it, I will replace the heatsink compound and use a much better lubricant. Hopefully then they will be good for at least another 6 months.
I think the internals of most GPU fans are similar so the same procedure might apply to many different cards.