You can't scrub oil and grease stains away. The trick is to draw them up out of the concrete. To do this, mix trisodium phosphate (or a TSP substitute) with water and an absorbent material to make a smooth paste. (See “Buying Absorbent Materials”.) The cleaner slowly soaks into the concrete and breaks up the old oil, and the absorbent captures it. Once the paste dries, the cleaning action stops, and you can scrape and sweep it away (Photo 3). Either throw it away or renew it with more TSP and water and reapply it for deeper cleaning. Use a nylon brush for cleanup (Photo 4). A wire brush may leave steel particles, which can cause rust stains.
Patience is the key. Old, long neglected stains may require two or three applications for complete removal. And even then, some stains might still show, or the freshly cleaned area might look slightly different from the surrounding concrete.
Buying Absorbent Materials
You have a variety of options for absorbent materials. For small stains, simply use baby powder or powdered talc. For larger stains, you'll need a bulk material. One good choice is diatomaceous earth, sold as a filtering agent for swimming pools. It's available from most pool supply stores.
Fuller's Earth is a finely ground clay that also works well for making a paste. But it's generally only available through chemical supply stores or mail order (www.sciencelab.com), and at $9 per pound, it's expensive.
Cat litter is also a good absorbent; however, it's too coarse to make a good paste unless you crush it into a powder. The same goes for the absorbent materials that are designed to soak up oil spills.
Cleaning old paint from concrete is similar to stripping paint from wood. Buy the same types of paint strippers. If you're working outdoors or in a detached garage (with the door open) and want fast results, you can use a methylene chloride–type stripper. The active ingredient will be listed on the can.
Caution: Methylene chloride can be hazardous. Use only in a well-ventilated area. And wear a respirator equipped with a fresh organic vapor cartridge (Photo 1).
If you use a safer, slow-acting stripper, expect to wait several hours for it to work.
Mix absorbent material into the stripper (Photo 1). Thick strippers won't need much. You can add more paint stripper to the paste after you apply it to keep it actively working on stubborn paint. Scraping with a hard plastic scraper may also help. But let the stripper do most of the work. After scraping, let paint residue dry and toss it into the trash.
Rust is really tough to get out, because no simple solvent will dissolve it. One effective way to remove rust from concrete is to dissolve the surface layer of concrete using a mild muriatic acid solution. (Muriatic acid is available at many home centers and full-service hardware stores for about $6 a gallon.) Working with an acid solution requires certain safety precautions:
- Wear rubber boots, goggles and rubber gloves. Also wear a respirator with a cartridge rated for acid vapor.
- Maintain good ventilation. Open doors and windows and add a fan for better air movement (Photo 1).
- Keep a 5-gallon bucket of clean water handy to quickly wash away any spill on bare skin.
- When mixing, always pour the chemical into the water, never the water into the chemical. And use only glass or plastic containers (no metal).
- Cover nearby areas with plastic to protect them from splashes.
- Read the manufacturer's dilution guidelines on the label and follow them if they're different from ours (Photo 1).
- Store extra acid out of reach of children.
The acid dissolves the cement in the surface layer of concrete. You want to minimize this etching process, so start with a light application and observe the results. You'll see the solution begin bubbling almost immediately (Photo 2). After the bubbling stops, scrub lightly with a long-handled nylon brush to see if the rust disappears.
Once the entire area is clean, thoroughly rinse it with lots of water to dilute any remaining acid and remove all traces of residue (Photo 3). Direct the rinse water away from your lawn or plants.
After cleaning with acid, play it safe. Thoroughly rinse your protective gear, wash your clothing and take a shower to make sure all traces are washed away.
Since each acid cleaning erodes the concrete surface, it's far better for your garage floor if you avoid the problem in the first place. Hang up wet tools and other items that'll cause rust. And protect the floor with paint or a sealer.
Note: If you have difficulty finding a respirator or mask for acid vapors at home centers, look for a dealer under “Safety Equipment” in your Yellow Pages.