A sheet of uncoated glass transmits ca. 90% of the light passing through it. The issue is the remaining 10% is mostly reflected and may produce ghosts of bright spots and bright areas. You can get some not super expensive camera filters that suffer much less of that, that are tested in the link I added to my previous amended post.
A thread adapter also allows you to test a much greater variety of filters, if you happen to suffer, as some of us do, from perpetual, possibly pathological, tool refinement syndrome.
But perhaps I am being too pessimistic and everyone coats their 1X, protective "barlow lenses".