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Removing plaster?

After parking my car in a public garage in another city, I noticed what seemed to be some type of plaster (cement?) droppings on my car, as if something dripped from the ceiling. Splotches of the stuff were on the sunroof, body of the car, rear window and the Prius’ back plastic fin. Of course, I didn’t notice any of this until I got home several hours later! I could easily wipe the stuff off of the body of the car, and scrape the stuff of of the glass. (It definitely is a chalky, powdery substance when it is scraped off) It is the stuff on the plastic fin that’s the problem. I first tried wiping with the rough side of a sponge using some elbow grease. Perhaps a little came off, but I stopped when I noticed I was scratching up the plastic pretty bad. I tried vinegar without success. I had heard a car talk story about using muriatic acid to remove cement from the body of a car. Could this be done on plastic? Anyone with any ideas out there for getting this stuff off?

The other day I descaled my coffee maker with vinegar, which gets rid of calcium deposits. Plaster is mostly calcium, I think…
Worth a try, maybe?
Edit: just noticed you’ve tried this. Sorry.

If it doesn’t work, add some oil and you got a nice salad dressing.

A claybar may do the trick but I’ve never used it on a plastic car.

Tom and Ray had a recent discussion on that, muriatic acid as I recall was the solution. listen here

I heard that episode. My question, what about muriatic acid on plastic?

Well, it comes in plastic bottles…

What’s the worst that could happen, you ask? The body dissolves and you’ll be the first on the block to have a environmentally friendly dune buggy.

Seriously, muriatic acid is quite corrosive. I’d be careful around electrical things. Hosing that car off afterwards might get some of it on connections and such.
How about hosing the car down and putting sandwich wrap on the offending spots? That may keep the moisture in long enough to soften whatever that stuff is. I’ve used the orange peel stripper or oven cleaner to strip body parts but the problem is that it dries too quickly so it takes a lot of elbow grease.
Wrapping it with ceran wrap, it keeps the moisture in and allows it to do work a bit longer and paint often just falls off. Maybe it will help you but with just using water… (don’t use stripper, clearly)

Id be highly reluctant to use muriatoc acid on a plastic bumper. While it comes in plastic bottles, there are countless polymer formulations and my guess is that mriatic acid will attack most. As a matter of fact I used to use a citrus based cleaner on bike parts, and I put it in a plastic cup once and it ate through the bottom.

I’d be incline dto either take it to a detailer or use “clay bar”, available at any parts store.

Try another type of acidic solution,such as Coca-Cola or seltzer water or even CLR-Kevin

Try vinegar, if that doesn’t work try ammonia, if that doesn’t work try a limescale remover with EDTA. Use plenty of water and rinse well after.

Takes something acidic to remove lime(calcium carbonate) a surfactant may help if its more grime then crystallized carbonate-Kevin

Muriatic acid is safe on plastic. Just use a dab at a time. A cotton swab is good. Do not pour it on. Do not use a clay bar at all. The clay will trap the sand and scratch everything. If you use a clay bar throw it away when done.