How to prevent a scalp burn from relaxer

Common cause of scalp burn is when the relaxer is left on the hair for longer than necessary.

When I did my first ever perm, the hairdresser applied some petroleum jelly on the tips of my ears and around my hairline before applying the relaxer. She told me it was to protect me from chemical burn. I thought that was sufficient.

However, she didn't apply any on my scalp. So at some point, I started to feel a burning sensation and I remember her telling me to persevere until I could bear it no more. I did, but my scalp burnt to the point that I had wounds with some hair sticking on the scalp.