we use a lot of simple green and 'something' blue at work for misc cleaning- Ive seen a LOT of damaged keyboard membranes on Fanuc CNC/robot controls minutes after wiping down with these cleaners- they seem to shrink up vinyl, especially if mixed 'strong'. I accidentally wiped my hands with simple green one day when we started up a machine, and went back to my programming terminal- by the end of the day the keyboard was discolored and cracked...with so many plastics under the hood, Id worry about anything that even possibly might cause any issues. The ONLY thing I use is fantastik or 409...wether cleaning circuit boards or engine blocks, it does the job. For cleaning circuit boards, far too many things theyve tried at work have caused corrosion over the years- I have NEVER seen any adverse effects from fantastik or 409(409 can tint stuff a little purple though). A buddy of mine that does a lot of CNC board repairs floored me when I saw him dunking PCBs into fantastik, but he was right(as usual), and Ive been sold on it for years. no residue, and dont attack anything, cuts dirt fast...when cleaning boards to repair I usually use a soft bristle paintbrush(bristles cut about half length) to help scrub, blow it dry and done.(none of the above are 'recommended' or perhaps usually warned against, but heck it works for me...). Just my 2 cents.
edit- as far as undercarriage, Im knee deep in mine right now...theres some pics here you might want to look at- dunno if I'm doing right or wrong, but this cars gonna have a fighting chance of surviving more winters than any other Ive ever had...I hope...I love this car!
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