I splurged in the art shop last weekend :-)
Trying Derwent's watersoluble Inktense Blocks
and a waterbrush - it has water in the handle
that you squeeze down into the nylon tip.
When its dry it's fixed, so you can add more drawing/painting
over your original layer without changing the original layer.
You can also use these on silk - but I haven't yet.
Links to Derwent's webpages for Inktense Blocks and Waterbrushes
There are lots of demos on YouTube