My normal solution for hiding any visible horizontal lines left during the printing process is Mr Surfacer. It's sold in various grades much like wet n dry paper. I use Mr Surfacer 500 mixed with cellulose thinner and applied with an old paintbrush.

Make sure you work in a ventilated area.

Mr Surfacer is available from Amazon and all good model shops (if you can find one).

https://www.modeldisplayproducts.co.uk/mr-surfacer-500/

This stuff appears remarkably similar to the old Holts Cellulose Knife Stopper which vanished from car accessory shops many years ago. You might even have some lurking at the back of your garage?

I have only tried this on FDM prints done in PLA. I have yet to experiment on 3D Resin prints. I don't know how printing resin reacts to cellulose thinner.

Cellulose thinner is still being sold in Wilkinsons stores at very modest cost.

https://www.wilko.com/en-uk/wilko-cellulose-thinners-125ml/p/0182560

Following a tip from Martin Brown, I can also recommend using Ronseal Multi Purpose White Wood Filler. It comes in 100 gramme tubes. Good adhesion. Very easy to sand and paint. Excellent for smoothing 3D printed hulls.(Or any rough castings in other materials). Use a finger dipped in water to do the initial smoothing then 800 grit wet n dry when the filler has gone hard.

https://www.diy.com/departments/ronseal-multi-purpose-white-ready-mixed-filler-100g/36121_BQ.prd



John