I have a concrete patio slab measuring 12' x 25'. Over 40-years, I've
used muriatic acid three or four times to clean the concrete.
About 6 months ago, oil-based paint was spilled in various areas on
the slab. I've tried removing the stains with muriatic acid and other
products (a solution of soap-and-hot-water; paint-remover; Ajax;
bleach, etc) but nothing removes the stains. What else can I try?
If the stains don't come out, can I paint the slab? (The slab has a
few hairline cracks). How do I prepare the slab, what kind of paint
should I use, and how would I apply the paint?
(I live in Riverside California (about 60 miles east of Los Angeles).
The lowest temperature may be 15 degrees and the highest may be almost
First, let's look at what you've already tried, starting from the well-known
fact that oil and water don't mix.
Soap and water wouldn't remove oil-base paint even if it was still wet.
Ajax and bleach work well on water-soluble things, not so well on oil-based
products. There are paint removers, and then there are paint removers. Some
work well; many don't work worth a damn. Chances are pretty good that the
People's Republic of Kalifornia doesn't permit the sale of any of the ones
that actually work. Look at the label of the paint remover you tried. If it
does not contain methylene chloride, it isn't going to work. Get one that
does. Examples include ZipStrip and StripFast.
Tried a pressure washer?
If that fails, sandblast it. A house in my neighborhood changed hands
recently. The new owner decided he didn't like the look of painted brick (who
can blame him?) and hired a crew to sandblast it. It looks beautiful. You
can't tell that it was ever painted (at least from the street -- I haven't
inspected it up close).
Oil-based paint, obviously. :-)
Roller. How long the nap needs to be would depend on how smooth the concrete
is; unless it's really rough, 3/8" is probably enough, maybe 1/2".
If you're subject to sub-freezing temperatures, I don't think I'd paint that.
Moisture's going to get into that slab, no matter what you do. I'm not sure
I'd want a layer of paint over the top -- too much chance of it spalling when
the temperature drops below freezing. Your best bet IMHO is to try to remove
the paint: ZipStrip first, then pressure washer, then sandblasting.
And put down dropcloths next time you paint. :-)
When you can't beat 'em, join 'em :o) My neighbor slopped dark brown
wood stain all over the concrete atrium floor in our condo. It is quite
old, and blotchy with stains, but the dark brown really stood out. I
took out my acryllic hobby paint, a stencil brush, and painted back in
the concrete colors - mostly gray, white, little umber, little yellow,
touch of blue. I painted in thinly a little darker color, for the
"pores" of the concrete, and then very sparingly a lighter tone for the
texture of the concrete. Not too bad, if I say so. Much better than
dark brown blotches, and you can't tell it's painted.
PS: Muriatic is not a stain remover. It is a concrete remover.
I doubt acid does well in removing things that are oil based. As
others have suggested, try a product specifically made to remove
If you decide to paint it, there are lots of paints specifically for
concrete. I recently used one of the concrete stain products from HD
or Lowes and was very happy with it. They call it a stain, but it
was fairly solid, covering more like a paint. I don't remember which
one it was specifically, but the color was New England Slate, which
might narrow down the brand. Prep on your concrete should be no more
than power washing it and caulking any cracks.
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