mineral spirits with a cleaning brush. personally I'd blast them with a pressure washer to save time.
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nice thing about mineral spirits is you can collect it after it has been used and bottle it. Let it sit for a long while and most all the junk in it will settle out and you can pour off your used mineral spirits for use again over and over.
edit2: I use a large basin I got at home depot to clean parts in. It's intended for mixing concrete/mortar but works well for this purpose as well. They come in a couple different sizes and are inexpensive. example here
edit:
nice thing about mineral spirits is you can collect it after it has been used and bottle it. Let it sit for a long while and most all the junk in it will settle out and you can pour off your used mineral spirits for use again over and over.
edit2: I use a large basin I got at home depot to clean parts in. It's intended for mixing concrete/mortar but works well for this purpose as well. They come in a couple different sizes and are inexpensive. example here
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