I thought when you got build up on the outside or any part of your tire it meant that you are pushing through the corners. "Pushing" though the corners will most of the time get you out of the groove anyways leading to the chunky build up on your tire. I use it to "tune" my car in a way. It is usually found when I first try a new setup on a car. I dont think you you will ever turn good laps when your tires gunk up they will just not be consistent at all.
Dont use motor spray.
Also make sure the side walls on your tires are always glued up. It will extremely effect the way your car handles in the corners. Check them often. Having two sets or more can really help.
Let the compound sit for as long as possible. I clean my tires after every run when I am done cornering. I plug in my battery then clean and sauce my tires. ( I sometimes sauce twice
)
Keep you tires clean all the time and in a sealable bag when you are not racing them.
Good luck
Chris