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Why do online sites say that polyurethane isn't food safe, and is there some way I can use it on my kitchen worktop and still be safe?
I don't prepare or store food directly on it or use it as a cutting board. I have it sealed with shellac now, but it's just not cutting it near the sink.
According to FDA regulations, the typical clear wood finishes that dry to a hard film, including polyurethane, are considered food safe. Wait to use the surface until the finish is completely dried, and clean it before allowing food contact.
Polyurethane is a fine choice to use on a counter, as long as you don't use the counter as a cutting board. If you do cut directly on the polyurethane surface it will be damaged. Mineral oil would be a good finish for a wood counter that is to be used as a cutting board, but you have to renew the mineral oil fairly often.
As you've found, shellac is not an appropriate choice for an area that will get wet frequently. However, you will not have any problem applying polyurethane over the shellac.
When I looked into this question in the past, I reached the same conclusion as JayL, plus one additional handy rule:
If I can smell the finish at all, it is not completely cured yet.
So when my thinned poly coats feel cured to the touch, I lean in and take a deep breath. Usually there will still be a faint whiff that lasts up to a couple more weeks.
This article says it is food safe only after it is fully cured.
According to finishing expert Bob Flexner, all finishes are food-safe once they have cured. Polyurethane varnish does not present any known hazard. However, no finish is food safe until it has fully cured. The rule of thumb for full curing is 30 days at room temperature (65- to 75- degrees F).
