> transparent varnish underneath had lead

Weird. Wasn’t lead typically used as an opacifying agent?

In any case, if you test by taking a little scraping, you can only detect lead that you actually scraped. But if you test with X-ray fluorescence, you get an integrated density over a decent thickness.

Also, even if the P100 respirator protects you adequately while sanding, it provides no protection against future exposure to the dust that you create.