I have wooden work tops ( all wood kitchen actually) it is some obscure wood and, on the whole, looks great
But, around the sink and in the grooves I have black patches. My friend has the same kitchen and hers is much worse but she is more chilled about it than me
I have started oiling around the sink more frequently and was tempted to varnish but was warned off by DIY shop
Tung oil is what I am using
I guess other than sanding down I cant get rid of it but any tips for preventing it getting worse or more appearing