I use a crc penetrant at the moment.
The acetone/atf mix works well, I’ve used it successfully on old machinery. I would warn though that it’s very harsh and strips finishes. If you get it on paintwork, whether it be one spot on the floor or on a vehicle, that finish will be wrecked because it causes blistering and the atf then penetrates underneath.
The best outcomes I’ve had with stuck fastners are when I’ve systematically gone around the day before and sprayed every bolt two or three times. On old fender bolts in vintage cars this is almost like a magic trick. It’s way too easy to snap the fastners if you just go at it. However, a couple of rounds with penetrant the day before often means I’ll get most out.
