When spraying the drawing done on Yupo paper with Lascaux fixative, is it safe to position the drawing on a vertical surface or should it be laying flat? Also, should one spray with several light coats or is it ok to apply the fixative with one heavy coat?

After you have completed your work on Yupo and wish to seal the artwork with a sealant like Lascaux fixative we recommend spraying the piece while it lays flat. Although Lascaux dries very quickly, you want to avoid any chance of the sealant running down the non-porous sheet.

As stated above you want to prevent pooling or running so a few light coats would be a safe bet. Due to its archival nature the sealant should not effect the color of the image or contribute to any yellowing of the substrate.