As far as redacting goes, these markers do a great job. They hide markings quite easily, although I do find that I need to use more ink when hiding pencil or pen marks. Printed ink isn't as difficult to hide. Additionally, I find it more difficult to use on colored ink.

Can I redact with a Sharpie?

Considering the price, the Sharpie Permanent Marker does a pretty good job redacting your sensitive information. The tips are chisel, which is optimal.

How do you reveal redacted text in a paper?

If you have a paper copy then there is no way to reveal the blacked out portions of the document. Look for layers left on the document. In GIMP, a free image processing program, layers are found in the right hand pane, under the header "Layers." Try selecting the black rectangles to see if they can be deleted.