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Concrete that is un-coated acts like a sponge to fluids spilled on it. When the fluid is water based the moisture will eventually turn to a gas and evaporate. When the fluid is oil based we have a problem.
Oil based liquids will only evaporate if the surface is treated with heat or sunlight (still takes a long time, months or even years) Therefore, interior surfaces contaminated with oil will be be especially problematic if you are trying to coat them. The trapped oil will act like a "bond breaker" or a repellent to coatings. This could be oil-spots or entire areas.
There is one solution if you desire to cap the unsightly contaminated floor, Oil Stop Primer. Oil Stop Primer, a (2) part epoxy, is coal-tar based. Therefore, the oil in your concrete will not have the same effect as it would on a standard primer or coating. It will act favorably enough to complete the adhesion process.
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Once the primer has cured the applicator will need to apply several top-coats of a polymer based coating to cover the primer's black color. The end-result is an attractive floor that no longer presents a danger to pedestrians.