trouble is TC, xara does not have a paint bucket tool [depite it's icon looking like one]- all you can do is fill an existing shape [or recolour a line] - separate lines that touch do not define a shape in xara, they are objects in their own right

you can get round this - I covered it in a previous thread, but PS is actually easier if you do not need pure vector

Do you get the white 'seams' if you disable the bucket anti-aliasing when you fill ? You might also need to tweak the threshold setting...