Although this painting of mountains works okay as it is, with a 'big sky' and spacious feeling, it is an ideal contender for one of those moments, particularly common in the Scottish Highlands, when the sun breaks through and casts rays of light onto the scene.
To create such rays of sunlight, begin by taking two straight-edged pieces of card and placing them in position, as shown (take care not to make the angle too large, or it will look like the beam from a lighthouse, or in this case an alien mothership).
Holding the card in place, scrub the surface of the watercolour, through the 'mask', with the Wonderbrush.

Note: The Wonderbrush should be damp with clean water.

Take care not to scrub too hard - with a little practice, it's possible to control how much you lighten the area - in this instance, we don't necessarily want to scrub right down to white paper.

Dab it dry with a piece of tissue.
And Voila!
For further rays of light...
... simply repeat the process in a different place.
As with all special effects, it is best not to overdo it. An effect like this should look natural and not too contrived.
The final version (although I think I preferred it with just the two rays of light!).