Help needed: How to remove "raster"?

I have a bitmap file that prints badly:

"Raster":     And closer:

I should probably draw it from scratch in a CAD system. But as a quick fix, I thought I could "fill in" the raster in Corel Paint (x8) to make the lines solid?!

Initially no filters work on black/white files - but after converting to grey scale they do.

But I simply can not find a filter that will make the "lines" more solid. Should be simple - but I clueless right now. Anyone kind and knowledgeable enough to give me a hint?