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I am really unsure where this question belongs, perhaps it is better suited for “Seasoned Advice” or “Home Improvement”?

I have a large cooking knife with a smooth blade. While sharpening it with a grindstone, I did a too low angle and scratched the side, like this:

Knife with scratches

Knife with scratches

This part of the blade now stick to tomatos and onions in an unpleasant way. I would like to smooth this out without damaging the knife any further.

Can I do this on my own or should I had the knife to a professional knife sharpener?

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    I rolled back to the unedited images as the resized ones did not show enough detail. – Willeke Jan 27 '18 at 11:30
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What I have seen from knife sharpeners, I would try it myself rather than going to them.

Use the softest tools you can find. And prepare for a long and slow process, to avoid further scratching.
What you need to use depends on the metal of your knife, but the list starting with the harshest you may need reads:

  • fine grained sharpening stone used with oil.
  • Copper or silver polish or tooth paste, (possibly with a mechanical buffing method.)
  • (green) scourer sponges with oil rather than water.
  • leather with very fine sand.
  • the jeans of the trousers you wear.

You do not need to do all in one go, you can do sessions between using the knife, as long as you are careful washing off all the metal shavings and the oil you used while working. Add a new coating of oil when putting the knife away. My grandmother used to always wipe the table knives with pumice, which while not as fine is as soft as you can get in stone.
If you get into that same habit with this knife, in the longer run the scratches will disappear. (Instead of pumice you can also use the (green) scourer pad.)

A yellow sponge with a green scourer patch on top
A sponge with the scourer pad, you can also get the pads without the sponge. Common domain.

The stone you can use is a natural stone, with a grain so fine you can not see it, so a smaller grain than any sand paper you can buy.

I would put the knife on a protective surface, like a cheap wooden board or some plastic sheeting with a couple of sheets of paper on the top.
Always use liquid while working, either water, spit or oil, and in this case I would go for food quality oil.

Make long strokes along the length of the knife with the tool as flat to the knife as possible.

  • I can see from the photos that this knife is hollow ground. No rigid tool will get the scratches, unless it's the same curve as the hollow in the blade. – Zeiss Ikon Jan 10 '18 at 18:39
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I'd try a buffing wheel using a red buffing compound.

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