how to best remove concrete splash from vinyl siding
If [the concrete] has set, soaking the concrete with a strong
detergent solution helps loosen its adhesion so you can scrub or
scrape it off. Georgia-Pacific recommends mixing the following
ingredients to make a strong cleaning solution for vinyl siding:
1/3 cup detergent 
2/3 cup trisodium phosphate  
1 gallon water
Scrub with a soft-bristle brush or scrape with a plastic paint scraper. To
give the scraper more leverage, wedge it underneath a concrete deposit
and tap it with a hammer; this usually makes the concrete pop off.
If you have a concrete deposit that just won't budge, muriatic acid is sure to dissolve it. If, on the other
hand, the concrete is spread over too wide an area to make scraping,
scrubbing or washing by hand impractical, you can opt for a pressure
washer, but check the siding product warranty to be sure you won't
void it by pressure-washing. Both options should be considered last
resorts.
Using Muriatic Acid
Step 1 Put on goggles and rubber gloves to protect
yourself from the corrosive effects of muriatic acid. Mix one part
acid with three parts water in a bucket to create a 25 percent
solution.
Step 2 Test the acid solution before using it. It's safe for vinyl,
but your siding may not be 100 percent vinyl. If it's safe, spread it
liberally in the concrete with a natural-bristle paint brush. Don't
use a synthetic-bristle brush -- the acid dissolves the bristles.
Step 3 Allow the mixture to dissolve the concrete. It may take several
minutes, and you may have to apply more. As the concrete dissolves,
wash it off with a garden hose.
Step 4 Clean off the residue with a kitchen scouring pad. Use the pad
only after rinsing to avoid dissolving it in acid.
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