So, I need to redact some information on my tax returns before sending them into my insurance company as proof of citizenship (or something, I'm not sure what exactly they needed them for). The problem is, I tried blacking them out with a dry erase marker and my SSN and financial info show straight through. I've tried going over them a few times and still nothing. I've run them through a copier, hoping it wouldn't catch the subtle differences in ink tones and would just show a black bar, but you can still see the SSN. How can I fully black out this information?
If you are able to make copies of the forms, create a digital copy instead. Then you can "black out" the information digitally with any photo editor or your favorite PDF software. Replace the target text with a black box or just remove it completely.
Then reprint the redacted copies as needed.
Bonus that you're not destroying the original.
Make a regular copy, then cut out the redacted information on the copy before you send it.
Put some opaque tape over the sensitive information before making your copy. You could try masking tape or electrician's tape. You might not want to use the auto paper feed on your copier because the tape might come off inside the copier.
Take a copy of the document, cut out the sections you want to redact. Put a blank sheet of paper behind it (optionally with black behind the holes, according to how you want the redacted sections to show), copy it again.
Or, of course, a digital equivalent, which is to edit by placing plain blocks over the redacted sections (do not use blur tools), then print out.
You can mess about with ink and whiteout and so on, and you can kind of squint at the result and think about whether or not you find it illegible. And you'll probably be right. But if you want to redact something, having in your pocket while the rest of the document is on the copier is the best way to be sure it doesn't somehow show through :-)
Also be careful about the size of the sections you remove. For SSN they're all the same, so this tells the attacker nothing. For financial information, knowing the number of digits in a redacted number might be a close enough estimate for whatever purpose they had in mind (and the number of line-items on your tax return is also slightly sensitive). Therefore where possible remove the entire box, not just the part of it you used.
I've found very few markers are up to the task of blacking out printer ink, but worry not, there is another way! When most people think of redaction, they think of blacking out data, but I've found whiteout to be far more effective. Just whiteout the data you don't want to send, and all will be well. If you really want it to be black, first apply a coat of whiteout, then just mark over it with your marker. Like the OP mentioned, it's also not a bad idea to run the redacted version through a photocopier, as even with the original there are a number of ways to get ink to show through, but it's much harder on a photocopied version.
Some copiers might have a mode for a pure black-and-white, non-halftone copy. Computer scanners nearly always have ("lineart mode"). If that is not available, physically cutting out the redacted information is definitely the safest way.
You can obfuscate each character individually with a ballpoint pen. Make sure the ink is the same color as the ink you're obfuscating. Then draw a shape over the top of each letter; I like using a figure "8" with an "X" through it. That will make every character look like every other character: an "8" looks like an "E" and like an "F", and also a "9" and an "X" and a "0".
DISCLAIMER: This isn't a perfect solution (obviously!). With enough time, someone could probably read all the information you tried to hide, especially if they used additional tools like chemicals, heat, high-res scanners, etc. But I've found that it's good enough for those day-to-day situations where you want to make it really hard to read certain text.
Cut out slips of paper that match the size of the boxes that you want to black out, then tape them in place with clear tape so that nothing is redacted that doesn't need to be.
Most photocopiers I've used won't scan words from behind another paper of standard thickness.
The variety of contemplations here are absolutely nuts, but I've (almost) been there. The simplest solution is a nice broad tip (1/8") felt tip pen (similar to a Paper Mate. Choose your weapon at an Art Supply store. Then double redact, scan and send. The Paper Mate type felt tip markers don't bleed through and the broader tip types you can find at the art supply give you better coverage for line-per-line stroke out of entire account numbers, transfers, store names, etc. I am redacting the Bankruptcy Court's request for all my bank records and tax returns. Even the Court trustee's paralegal said it was "a matter of law," that I redact my information before transmitting it even to them... something my own attorney didn't have the acumen to tell me. Invest in redacting tools like these markers, blackout stamps, and have fun. Think of it as an electronic shredder.