In my experience thru the years of doing airbrush tanning and now HVLP as well the area of the breast's is a delicate situation.
The pores here can actually be on the larger side on some women, and if this area gets sprayed to heavily with solution can leave the skin looking like it has blackheads the next day. NOT quite what you had in mind I'm sure is it??
Your part: scrub this area very well several times before your session, you can also treat this area with some witch hazel or a gentle toner too, to help with pores an hour or so before your spray tan. This will help tighten them some and you can also use a cold washcloth then follow up with the toner to help close the pores. One other thing that can lead to splotchyness here is women who directly apply perfume to the area right above breasts and between. Spray your clothes NOT your skin and that should help.
Technician: The person spraying you should take into consideration your pore size on your chest and face area and not spray to much or spray a little father away. If pore size on the face is larger you can bet the skin between the breasts is similar and adjust accordingly.
Peeling spray tan: This can be a combination of things, not exfoliating well and on a regular basis and possibly product or oils on the chest prior to spraying. If you don't keep up with lotion as well it can make matters even worse. I know great right?!
Doing a prep right before your session will help remove any product residue and natural oil accumulation from the skin as well. Ask beforehand if the place your going to offers a skin prep like I do before they spray you. Especially if you have had this problem in the past. Communicate that with the tech spraying you so that she knows and can adjust her technique.
Down Under...Last but not least is spraying under the girls, this is also a tricky area as the skin on skin contact will leave the solution in a moist place where it can't properly dry like the rest of your body. I usually spray just once underneath as I would rather it be slightly lighter than "orange" or have streaks if you spray to much. If you are large breasted I suggest wearing a strapless old bra to cut down on any issues.
Hope this help you!