A few points- Wear protective gloves because your hands are in the paint vs using regular brushes with handles.
1 – The under-painting is important so think about which colors you want, but be creative – the colors in video are kind of dull.
2 – Don’t use medium in the under-painting or the second layer will tend to lock down too quickly when you try to erase through.
3 – Keep ‘spritzing’ with water before, during and after.
4 – I like white vinyl erasers best for this. You can slice and shape them, half-cut and tear them. I like the ‘rubbery-feel’ vinyl erasers give. This can also be done with cut credit cards, but with a different feel during execution. Try both!
I love doing this and try to get a ‘geological’ effect. Later, you build up glazes of blue-green-white in the water but aim for some transparency by lifting some blue glaze areas with wet paper towel to reveal warm-ish underwater rocks. I also do some more cool glazing over the cliff faces to push parts back in space. – Also see “Eraser Aspens” post . (This technique can be done in watercolour by first coating the paper with a 50% gum arabic and water solution to ‘seal the paper’ and allow lift outs, but credit card fragments work better than erasers for WC on paper.) A few more eraser-painting samples below. These were all in-class demonstrations. Good luck!