How to make Line Art - lines thicker in Photoshop:

Once you have cleaned up the drawing and made everything as black as possible. You can even convert the file to bitmap and back to grayscale at this point. The key here is to have the file as clean as you can get, no little dots or spots of black in places you don’t need them. (Because when you expand the lines you will see all the marks you missed and will have to clean it up then.)

1. Select then Color Range use the eyedropper to click on the blackest part of the drawing, and then set the Fuzziness to 200 then click Okay.
2. Next go to Select then Modify-Expand depending on the dpi of file you need to expand the selection if the file is 800 dpi I would do expanding of 4 if higher than 800 dpi you will have to do high but you shouldn’t go higher than 6 or 7, remember you can always go back and tweak things later.
3. Now you need to zoom into your image you should see that the selection of your drawing has expanded into the white, that’s good that’s what we want. Now go to Image-Adjustments-Replace Color, click the on the white part of your selection (the white part closest to the black of the drawing) then at the bottom of the Replace Color window I want you to adjust the Lightness marker all the way to the left making the white part closest to the black, black as well, make sure the Fuzziness is set at 200 click okay.
4. Now Zoom out and if looks thick enough then your done, if it doesn’t then repeat the process until it does. If it looks too thick undo and repeat the process again making sure you don’t expand the selection as much.
5. Save file as Bitmap 800 dpi tiff or whatever you file can handle.

So there you go, to can now use this trick on anything from deleting a little bit more to adding just hair line of color around any object.