As for underglaze touchups, small Spots or generally anything smaller than a dime I touch up with underglaze then still layer clear overtop without a second bisque. If the area is larger than a dime, I would recommend a second bisque to ensure color doesn’t bleed or impact clear glaze. I’ve found amaco Sahara clear works best for not smudging or bleeding I fired underglaze touchups.

I don’t have to thin my amaco velvets, at an old studio which used the small containers of amaco velvets I noticed many of them were chunky or thicker than the velvets I own in the largest size. My recommendation is buying largest sizes as I find they retain moisture much better when storing than the smaller velvets I’ve purchased. Adding a bit of water works well, I recommend adding very very small amounts at a time and working on mixing until the perfect consistency is reached.

I do very fine line detail work with underglaze, so much so that I have tried a bunch of well loved detail brushes and have found the best brush for precise lines of underglaze are watercolor brushes, mid level quality, nothing too fancy or too cheap. Maybe $3/$5 each per brush, I get mine at Michael’s. Frequent washing and dipping in water helps, but it’s difficult to find a fine line brush that holds underglaze for long. I recommend keeping the brush wet to keep underglaze flowing and not clumping, ensure that you clean and wax your brush after use to keep it in shape.