Thanks, i will try it soon, I am just at the beginning phase of learning, but find many interesting things.
For now, it looks that reducing opacity on fill layer and painting on it could solve this, thought I still don't fully understand how things work with masks, while Affinity looks very flexible, offering alternative options, which is really promising for my way of working.
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...oh, I tried it in the meantime and that's exactly what I needed, as simple as pressing Q, but hard to find without some help or reaching the back cover of help index.