You just keep on and on and on using up swathes of abrasive paper and slinging them clogged, and hardly used to the max.
I had the same issue when sanding down woodwork recently. The first coat of whatever, be it stain, varnish, paint, ??? did precisely that. Strippers were of minimal help although I did, at the time, find other chemical removers that did help. But ultimately even if they get the worst away, there is still residue to remove and the answer seemed to be to curse having started, and keeping on going.
Accept the time and cost, take a deep breath, or a thousand, and keep going. Tell your friend to think of how good they'll feel once it's achieved.