This set up will allow you to paint out hard to reach advertising messages, create block letters, or horizontal or vertical stripes.

You might be thinking to yourself "big deal!" Well...

Yes, something like this probably exists commercially. And it's probably over-priced.
Yes, the G.R.L., Barry McGee, and Citizens Against Ugly Street Spam have made similar tools.
Yes, it's not challenging to make and hardly requires instrcutions.
Yes, it doesn't take a genius to think of this and you probably would have come up with it yourself.

BUT, if this is what you need, it's an awesome tool that you can make for at most $6.
Also, at $6 it can get lost or become disposable in a pinch.
Once you know this tool exists and how to use it, hopefully you will be compelled to make one to paint over hard to reach advertising and beautify your neighborhood like someone I might have heard about maybe did one time. It's quick, easy, and fun.

Step 1: Get Your Materials

Everything you need can be bought at a hardware store, though some should be in most garages or workshops.

1. A wood broom/paint stick. Or any stick you can drill into. The one I found was about 5 feet long and $3.99. Perfect for what I needed it for.
2. Foam brushes. Found in the paint section. I hear foam brushes are good if one was to often walk around painting out advertising and couldn't always clean their brush. Each one costs around $.75 and I try to use them a few times. Get the ones with wood handles, not plastic.
3. 2 screws
4. A small can of black house paint.

Bonus:
1. Duct tape. This can help support the brush and keep it from moving. Especially if you shear the head of the screw because you didn't pre-drill.
2. a drill bit to pre-drill your holes. I didn't have one handy, so I drove the screw straight in, cracked the handle, and sheared the head of the screw. Not ideal, but it is called the "Quick and Dirty Low-Tech Ad Buffer" isn't it?

Step 2: Attach the Brushes

1. Pre drill a hole through the brush handle and into the broom/paint stick. About 3 inches from the top of the stick.
2. Drive a screw through the hole you drilled.

Orient one brush horizontally and the other vertically. This way you can draw horizontal and vertical stripes.

Note that each brush is NOT perpendicular to the broom/paint stick. It's set an an angle so when you lean the stick against the wall the facing edge of the brush and the wall will be parallel.

Step 3: Rock Your System

Now you are ready to beautify the neighborhood. Fill a resued food container with cheap housepaint and hit the streets.

Why a reused food container? If you leave it outside while you catch a drink at a local bar, no one will touch it. Your paint will still be there when you come out to walk home and buff more ads.

Ok, so you're walking around with your stick and your little container of paint. You see an out of reach ad you'd like to buff. Dip your brushes, extend your stick and you'll have that nasty ad covered in no-time. Someone looks at you funny, or you see a cop car pull a u-turn? Drop your stick and walk away - that $6 isn't worth the hassle.

This can be used to quickly buff billboards, posters, and other forms of outdoor advertising. Sure, it's not practical to cover large areas, but you can black out product names and images so the ad looks more like those self-censored CIA COINTELPRO documents. If you really want to go vigilante style, learn what advertising in your urban locale is illegal. Outdoor advertising usually requires permits, so most wheat-pasted poster advertising applies. Check your local laws.

Oh, and remember, you don't even need the stick to work at street level, so get out there!

See also:
NY Press Article on Illegal Advertising
Anti-Advertising Agency
VisitSteve.com