Susan......I would not have don that - and melamine paint was not designed for counter tops - it was actually developed for cabinets. You must be in Canada....no?
Melamine paint will yellow - it is oil afterall - and all oils yellow somewhat.
Do not put an oil based urethane over it - that won't help protect it - and it is just one more product not designed to be on a counter - that would likely give you problems.
Just be kind to your counter tops - no cutting, strong cleaners, or scratch pads - and they should be ok........
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