Hello and welcome to healthcaremagic
UV A part of the electromagnetic spectrum is mainly responsible for
tanning.
Therefore, a good sunscreen should have broad spectrum UV A coverage in addition to UV B coverage, in order to be able to prevent tanning and it needs to be applied in sufficient quantity and every 2-3 hourly.
A sunscreen with minimum 30 spf would be adequate. It should mention both UV B and UV A coverage. Therefore it is important to select the right sunscreen for desired effects
It should be applied in the following quantity:
--3ml for face and neck(a little more than 1/2 teaspoon)
--3ml for each arm(a little more than 1/2 teaspoon).
A sunscreen should be applied every 2-3 hours.
If I was your treating doctor I would have asked you to use a cream containing one or more of the skin lightening agents e.g
kojic acid,
glycolic acid, arbutin or
hydroquinone etc once daily at night to gradually fade away the tan.
I hope you will find this information useful.
regards