Best way to clean plastic splash shields, cowl screens

What is the best method and cleaning materials to return the black plastic splash shields (headlight, fender, frame) and cowl screens to a like-new appearance?

Without resorting to armor-all or other plastic dressings or dyes...

Thanks in advance

When talking about ABS or Polyethylene parts a soak in warm caustic soda soloution works well to clean them. To bring back the original lustre after it becomes dull or discolored from age/UV degredation, etc. there's is no way to do that, once the plasticizers in the material have changed or degraded it's not reversable. An application of topical dressing is the answer, and it will likely have to be reapplied from time to time to keep it that way depending on the environment the car is in.

So just how caustic? Lye/sodium hydroxide caustic or? Suggested products?

The black plastic is still quite pliable and appears to have good depth of colour. However, some of the pieces have residual undercoating and/or overspray which I'd like to remove without resorting to scraping or blasting, which replaces one set of problems with another

My ..........

I've actually had decent luck letting them soak in a 'bath' of that "Purple" cleaner (under

various brands) . Especially if it's had overspray on them from a previous 'quickee'

paint job. I soak for couple days and go over with a soft brush ...

My cowl screens had some over spray on them from a previous repaint. I soaked them in old brake fluid to remove the paint. They were slightly discolored so I misted some trim black on them. Not OE but they look good. They are in a dimly lit area so you can't tell there's paint on them.

After power washing the undercoating off the splash shields, they were in good shape. They will get sprayed with more undercoating so I didn't go crazy with them.

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So just how caustic? Lye/sodium hydroxide caustic or? Suggested products?

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I bought the stuff I use in B.C.Canada at a company called Magnus industries, it was many years ago and came in powdered form to be mixed with water. I think they sell it in liqyuid form now. Any machine shop should be able to get some for you. It works especially well to remove paint off of plastic.

As a backup keep in mind that Polyethylene/Polypropylene plastics (which is what those two items happen to be made from) is HIGHLY resistant to most acids and chemicals. in fact most of those chemicals are sold in containers made of PP/PE. So pretty much any chemical you use will be safe on those parts.

As an aside, if you are unsure of what type of plastc parts you are working with ALWAYS test a small spot first, all plastics are NOT created equal!