Black, marked wooden kitchen worktops.

[DanielleandBobby]

Do I need an orbital sander to tackle this? Just that really. Can't afford to replace all the worktops in our kitchen but they are looking grotty (even though they're not dirty, just black). Thanks.

Black with what? Some marks will go pretty deep and won't sand out, but yes any sort of sander will do. Start off with a fairly heavy guage paper then go lighter and lighter. Once the wood smooth and clear you need to oil it but don't do what I did and oil it after rough sanding or it will all go into the opened grain and just look awful.
Danish oil is supposed to be the best but needs at least 6 hours to dry between coats - probably overnight really. Then it needs redoing every 6 months.

@Fizzadora thanks. Not sure why they're so black - I suppose it's just water soaking in over a long period of time. It's around the sink and drainer that it's the worst.
We inherited the kitchen when we moved in just over a year ago. I guess the kitchen's about 5/6 years old so they probably haven't been looked after properly. I'll have a go with the sanding and oil - thank you :)

I recommend osmo to treat them once you've sanded out the marks. Repeat yearly and they should stay looking beautiful.

Agree with pp Osmo is amazin and our worktops look really good. my brother has this on his and does them every 2 years near the sink but leaves the rest and it looks as good as when he first did them
I used clear matt. be careful not to get the one with white in it as this can look cloudy

Before you start doing anything it might be worth giving them a spray with Astonish mould and mildew blaster.

groceries.morrisons.com/products/astonish-mould-mildew-spray-402623011

What you need OP is oxalic acid, look it up. You can buy a bottle of it in builder's merchants. Sand off any finish and treat with the acid, it's specifically for bleaching black marks in wood. Then after you can re-seal with a good quality wood oil. I love wooden worktops. After the hassle of sanding mine the first time I never let them get black again. You need to oil 1x every 6 months. If you do this, it takes literally minutes, you can guarantee beautiful worktops. Sanding with an orbital sander is a jeffing nightmare. My kitchen was literally like you'd thrown a bag of flour into the air. Good luck OP

Sounds like the water has created mould and is probably quite deep. I'd try a patch test of sanding, but think it is probably beyond saving.