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Bladed scrapers will also do the job - but the card is more effective at getting the pressure and width of coverage right.
You don’t have to get all of the finish off but the more you do get off the fewer sanding pads / paper will be sacrificed.
Went through 5 80g 150mm pads on this lot. + 2 120g + 3 detail sander pads
Howard-
This is probably a bit late however; you will go through considerably fewer sanding sheets if you cut a small slit in the arrangement that connects the sander to your sacrificial Dyson. Dedicated dust extractors have a small "window" that can be opened to prevent the suction from it pulling the sander onto the surface harder than it should be while not effecting the performance of the extractor. This helps to prevent the pad heating up and not only prolonging the life of the sheet but also the backing pad on the sander itself.
Bobbo
This may inspire me to actually finish the job, never heard of a card scraper before but I can see i've been desperately in need of one - once all the old varnish is off I actually quite enjoy the final sanding by hand but that looks just the ticket