As you can see from the photo above, the tagging was really extensive. Restoring it was a daunting task--I have never tried to fix a mural before, let alone one spanning about 40 feet across and 10 feet high. I even researched on the internet for tips and guides on how to restore, but there really were no clear cut guidelines that I could find to reference for help. It isn't a perfect method, but here is what I found to be the best way to fix an outdoor mural painted on a concrete surface that's been defaced with graffiti. Also, it is important to make sure you have permission to fix the work up. If the mural belongs to the city or an organization, contact them first. This mural was done by a local resident years ago and is not affiliated with Mural Arts of Philadelphia.
Before and after picture
Goof-Off or similar graffiti cleanup spray can be effective to remove the spray paint. You can also purchase environmentally friendly paint remover in large quantities if the paint has been dry for too long and something stronger is needed. I have tried many different products to remove paint, and the easiest, safest and most effective so far is Motsenbocker's Lift-Off, which is also biodegradable and water-based.
Clothes you don't mind destroying
Coarse scrub brushes
Coarse scrub pads
Small, sturdy plastic trays (frozen dinner trays/takeout trays work great)
Medium or good quality acrylic brushes (Sturdy, synthetic bristles with flat or filbert head)
A few regular sized house paint brushes
Paint rollers and trays if needed
Buckets with warm water
Acrylic paint - some can be the tube or tub kind you get at the craft store (these colors are brighter and the paint is thicker) some can be the gallons and half gallons of house paint (the color will be more subdued but the paint goes further--good for large areas of block color). We used both depending on the color.
Clear sealant/non-yellowing topcoat (for easy removal of future defacing)
I decided we needed to remove as much of the spray paint as possible because we really couldn't see the details of what was underneath without at least trying to strip off a layer. There was a lot of detail on this black and white striped globe, pictured below.The bright tag with black outline on the left was done with silver spray paint, which had to have been the most annoying color to remove. It has a slimy texture and it gums up when the paint remover is applied. You shouldn't need to use a whole lot of effort to see the graffiti begin to vanish.
When you've removef all the graffiti you can, let the surface dry for 15 minutes and begin the painting process. Don't be discouraged if there are layers you can't get off; that is when painting over the damage comes in!
Starting with the closest color you have on hand, you'll then adjust the color and saturation. For this blue, I started with a sky blue house paint and toned it down with white, and added a bit of bright yellow to bring it to a slightly more sea-foam color. Paint is always lighter when it is wet, so it should look a shade or two lighter than the color you're trying to match.
If you can't match the paint exactly, consider painting over it with a similar tone that you have on hand. Some colors I didn't have (navy blue, brownish-maroon) so I just matched with what I had (dark blue, and as close to maroon as I could get). Look up "color theory" or "how to mix paint colors" for a basic guide. (And throw out that red/yellow/blue theory while you're at it)