When you wet sand I would use water as a lubricant rather than lemon oil. The lemon oil would make it difficult to apply another coat if you do sand through. It's best whether you sand wet or dry not to sand the corners but to sand up to them. Then with a pretty worn out piece of sandpaper just scuff the corners a little. It's too easy to put too much pressure on the corners because of the narrow surface so much care is needed to sanding there.