I'm amazed at the amount of people who wouldn't do it because they think the client might bring in dodgy solution. I can understand those saying about the fiddle of swapping solutions and cleaning the gun etc... as this means it will actually take longer than normal to do the tan.
I wanted a spray tan for a wedding I was attending at the beginning of the month and I wanted it to look nice of course, I have the solution I know looks lovely on me & it's an organic fast tan one so I can wash it off quickly. Being my solution I know how long to keep it on, I know it's stored properly, I know it's a quality solution and I know it looks nice on my very, very pasty white body as I've had my boyfriend spray me with it once before (he is not great at it yet though as he's only sprayed me twice).
I considered ringing a local therapist to see if she would spray me with my solution for all the reasons above (I know she uses Sienna X which I don't like) I was hoping she would have the same machine as me (5008) as it's a popular machine, that would have meant I could take my pre set gun with solution for her to spray me with and I wouldn't have expected it to have taken her any longer than normal to spray me and then she would have just swapped the gun back to her own. I was totally willing to pay the therapist her full price to spray me as I was wanting a professional spray tan.
I ended up just letting my boyfriend spray me for the second time and hoped it would turn out ok. Thankfully it wasn't too bad, he did get my knuckles on 1 hand and both my wrists were a bit messy but I sorted it with a microfibre cloth and I looked ok for the wedding. It wasn't as good a tan as I would have had from a professional as there were problems but I wasn't pasty white and there wasn't any major isues with it.
Maybe the client has a good reason for using their own solution. Personally I would want the background information of why they want their own solution and if they have tried what I use or not. I wouldn't just assume the worst of the client, (that they will bring out of date solution, they may have watered it down or contaminated it etc...) if there was no valid reason for using their own I would think twice about doing it because of the added time to clean the gun befoe and after the tan etc... but it could be a silmilar thing to what I wanted - a fellow therapist that wants their own solution for their own reasons (including advertising their own solution).
Basically I would want the full reasons for wanting to use their own solution before making any decissions.