I'm seeking thoughts about a paper that is thin and acid-free. I will be using this paper in collage. It needs to stand up to bring totally covered with water-based paint and not curl too much. Also need a variety of sizes. Thoughts?

All the papers in our collection are acid-free. The thinner the paper the more susceptible it will be to curling. But this also depends how heavy the paint will be applied, We suggest to use a heavily sized paper such as the Stonehenge Aqua. This is available in both a textured Coldpress and a smooth Hotpress, The lightest weight is 140lb (300gsm). Here's some more information about it:

https://legionpaper.com/stonehenge-aqua/