Answer: There are several ways to remove oil stains. Your pressure washer is a little low on the pressure to do a really good job (most contractors recommend at least 3000 psi), but it is worth a try. Using a rotary nozzle instead of a standard fan nozzle can increase cleaning speed. Fan nozzles deflect the water on an angle, which slows the water down. Rotary nozzles, in contrast, spin an undeflected water jet in a circular path so that the water leaves the nozzle with greater speed.
Also, try adding a little degreaser to the washwater and use hot water if possible. Hot water can lift oil off of the concrete, especially in cold weather.If this doesn't work as well as you'd like, try a poultice of trisodium phosphate (TSP). Mix one part TSP in six parts water. Apply to the stain and let it sit until dry (20 to 24 hours) and, then sweep it away. This will remove most of the stain.
If you want to get more aggressive, try mineral spirits or paint thinnerùapply until saturated then cover with an absorbent material, such as cat litter. Let stand overnight. You can even try making a poultice of 5% sodium hydroxide. You can also try bleaching the surface with laundry bleach. Note that these last two are somewhat dangerous materials so be careful.
Another option is the use of an alkaline degreaser. Hot water lifts the oil from the concrete, and the degreaser emulsifies the oil, allowing it to be flushed from the surface.
You can seal the surface to make stains easier to remove in the future. Use a good quality penetrating sealer (silane, siloxane, or high-molecular-weight methacrylate). Get recommendations from a local construction supply house for brands.
If you have any further questions, or want to offer you neighbor some current literature on how to clean concrete and masonry surfaces, the Hanley-Wood Bookstore has just published Cleaning Concrete and Masonry, a reprint collection of the articles we have published on the subject during the past 20 years. To learn more about the book, visit the bookstore Web site, www.hwbookstore.com. Copies will also be available at the World of Concrete Bookstore located near Hall A in the Morial Convention Center in New Orleans this month.