Here is what I did and how it worked for me!
1) I met with the owner. It took a little to find out who he was but was able to get his phone and arranged a meeting
2) I asked what his off-peak time was, so I could coordinate coming over then
3) I did ask if I could use an electrical connection and offered to compensate him for the cost of electricity (no matter how nomimal it was)
4) I asked if he would have any problems if I came to use the facility and stay a little longer to finish the processes (which I explained to him in grave details)
At first he was very reluctant but I offered a point which he could not deny, which is people would talk, stare and come over if they saw me doing a full job on my car.... which they did. I also offered the possibility of coordinating (during off peaks) with friends to come over, which he understood the marketing value right away. For those guys, an empty place is not good for business, and a busy (even just busy looking) place much better for business. In addition, one person leads to another and so on!...
I have to admit that I sold it goooooodddd! and repayed his generosity in spades. I guess he was happy to have me onsite (more often then not) checking on the place, and constantly rinsing the bays and racking up the wands, and picking up trash. Not to mention I would be at the place late evenings. It did help that he came to see what I was doing the second week of our agreement, and I was sweeping the place

He ended up giving me a key to the storage area where I kept my extension cord and some of my products and never charged me for electricity.