For texturing the stream surface, I used a technique suggested by Luke from APS/Geek Gaming Scenics, using a mix clear sealant and white spirit.
The sealant was Everbuild Clear Fix. I mixed in about 1/3 by volume white spirit and worked it for a while with a wooden spatula – it took a while to get them to work in together to a smooth paste, but that did make it easier to work and stipple. Basically I followed the instructions in the YouTube video just added a thin layer and caught the surface to give it a more natural surface. The only downside is that, being sealant, it doesn’t dry hard like the resin does – it dried to a rubbery hardness. No idea at the moment how durable this will be as a surface, but I am hoping that even if it gets damaged, it should be easy to repair.
Two things that I have retrospectively noticed:
Firstly, the surface of the sealant attracts dust etc quite easily. This was especially noticeable when sanding down the resin bulge – I should have done that before I added the sealant, not after.
Secondly, I realised at this point that some of the clumps of field grass that I had put in very densely clumped had pulled up a lot of resin via capillary action, which had then set and made them quite rigid. So while most of the clumps were realistically flexible, there are some that are quite hard as well.