Many old school watercolor books I've found are written by purists who use transparent watercolors only, and look down their noses on any use of opaque media. I get why they love the fresh, luminous look of transparent paint. But coming from oil painting makes staying in the transparent zone easier said than done. I'm used to the option of painting light over dark. And you can't do that using transparent paint. Light colors just won't show up over dark.
Nevertheless I persist. I continue practicing transparent techniques to get better. But sometimes, after I've already tried and failed, it's time to save my work ... by any means necessary! And why not. It's kind of liberating, actually. Besides, after a recent trip to the Metropolitan Museum I couldn't help noticing that many of the modern and old masters made drawings and paintings in mixed media. Watercolor with charcoal, watercolor with pastel, watercolor with opaque white, watercolor with ... you get the idea. I'm by no means a watercolor historian so I'm not sure when the cool kids got so pure. But I'll look into it and get back to you.