Very Smooth Surfaces Techniques (inside tube)

I’m hoping to get a very smooth surface inside what is essentially a tube that opens up (think: pumping liquid through a loop, and making one segment “open” to allow for dissolved solute sensors to be inserted into the tube). As this shape will be fairly unique, we are printing this component; however the liquid medium will have algae in it, which requires smooth surfaces to mitigate biofilm buildup.

Using two-part resins with a thinner, like Acetone, sounds like a possible solution; however I am very new to using this medium and wanted to see if there are any (1) application methods, (2) specific recipes, and (3) general warnings that the good folks on this forum could provide before we start experimenting away.

Would appreciate any advice on materials, application methods, or anything else that could be helpful for this process.