[Techtalk] GIMP: Changing all set pixels to black

[David Sumbler]

In one of my GIMP projects I have layers which contain outline shapes.
These are from .bmp files, and unfortunately there has been some
antialaising (or dithering - I'm not sure what these terms mean
precisely), so the pixels in the outline are not all black, but various
shades of grey.

I want the outline to be entirely black so that I can then fill the area
with black and then convert the solid shape to some other colour.  The
area outside the shape will be transparent.

I have tried various things that I found on the internet for changing
all colours to black, but they do not seem to affect the grey pixels in
the outlines.

If I use the colour picker on one of these grey pixels, it actually
produces black, so I think that they are not really grey at all, but
black with some degree of transparency.

The answer would seem to be to remove all transparency from the layer at
the outset.  I can then convert the area outside the outline to full
transparency at a later stage.

Can one of our GIMP experts tell me how I can get the result I want?

David