Rust marks on kitchen sink

We have a white sink/drainer which was about a year old when we moved in about a year and a half ago now.
It appears to be made of some sort of plastic or fibre glass but in the last six months rust coloured specks have appeared in the bottom of the sink.
None of the normal cleaners including bleach have any effect on the marks.
As we have never left anything metal standing in the sink I wonder if this sink unit is metal covered in some sort of plastic and is rusting internally.
Has anyone else experienced this problem or know what can be done about it.
Thanks Roger
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