Just my thoughts. Using a selection (made somehow - color select / fuzzy select ...)
Pre-process using the quick mask.
1) Bottom of the Gimp window is a Quick Mask toggle icon.
2) The default colour is 50% red, if that is difficult to see, right-click on the icon (3) to raise the Quick Mask Attributes and change color / opacity.
4) Paint out selections with a brush and white FG.
Post-process using the eraser.
The convention is use a white BG colour. (used by the alpha mask) Use the eraser tool to clear parts of the image not required.
Obviously for small areas close to the selected area, either way, zoom in and use small brush size.
After using say the Foreground selection tool, which typically leaves plenty of semi-opaque border pixels, I often use the eraser tool, small fuzzy brush, and click -click shift for (short) straight lines around a border.
Pre-process using the quick mask.
1) Bottom of the Gimp window is a Quick Mask toggle icon.
2) The default colour is 50% red, if that is difficult to see, right-click on the icon (3) to raise the Quick Mask Attributes and change color / opacity.
4) Paint out selections with a brush and white FG.
Post-process using the eraser.
The convention is use a white BG colour. (used by the alpha mask) Use the eraser tool to clear parts of the image not required.
Obviously for small areas close to the selected area, either way, zoom in and use small brush size.
After using say the Foreground selection tool, which typically leaves plenty of semi-opaque border pixels, I often use the eraser tool, small fuzzy brush, and click -click shift for (short) straight lines around a border.