As long as you let the ink dry first, it will definitely cover the fountain pen ink. But if you try to apply it while it’s still wet, the ink may mix into the white-out and form a weird smear of color. The other main problem is that white-out can get stuck in the nib and clog it. Correction tape tends to work better, but I’ve still had some of those issues with some brands that have a chalky texture on the surface of the tape. I’d recommend using a different kind of pen over top of white-out, if you’re going to use it. Personally, I just got tired of the hassle and started crossing out any mistakes that were too egregious to write over with the word I intended. Flossing tines to get white-out unstuck from between them is not my idea of a good time.