As long as you clean them with mild soap & water the original sealer should last for years. Only if you see water spots or dull mark does it need to be sealed. You can but a "natural stone sealer" from Lowe's or Home Depot. If you try to seal to often it will become hazy and need to be cleaned with denatured alcohol.
Yes they do but you can do it yourself. It's just a matter of spraying sealer on now and then. We have quartz. No maintenance at all.
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