Wet on wet is the technique that makes watercolour feel like watercolour. Paint dropped onto a damp surface moves, blooms, and settles in ways that cannot be fully controlled, and that is precisely the point. But most beginners encounter it through short videos that compress ten minutes of waiting into ten seconds, and arrive at their own paper expecting something that flows. What they get instead is a grey-brown wash that refuses to lift.
It is almost always about the shine
Before you touch a loaded brush to wet paper, look at the surface from a low angle. If there is a visible sheen, the paper is too wet. Your paint will flood and crawl. Wait until that shine has just gone: the paper is still damp, still receptive, but no longer slick. That window is where wet on wet actually lives.
The problem is almost always timing, and timing is the thing no short video can teach you. The window between too wet and too dry is narrow, and it changes depending on the humidity in the room, the weight of your paper, and how much water you laid down. You have to learn to read it rather than follow a rule.
Why paper weight matters more than technique
On 90gsm cartridge, wet on wet is essentially unworkable. The paper buckles, the water pools in the valleys, and the paint chases the puddles. On 300gsm cotton paper, properly stretched or block-mounted, the surface holds moisture evenly and gives you time to think. This is not an argument for expensive materials before you are ready. It is an argument for understanding why the technique behaves differently depending on what is under your brush.
Where to start
Begin with a single colour. Wet a small area of paper, wait for the shine to pass, then drop in a wash of French ultramarine. Watch what it does. That is the lesson, not the result, but the movement. Wet on wet is a conversation between water and fibre, and like any conversation, it rewards patience more than speed.