Can you apply epoxy vertically?

No, with rare exceptions. If your epoxy project will be vertical, you'll need to first apply your epoxy coats while the surfaces are horizontal.

Afterward, allow those surfaces to cure for 72 hours. You can then hang or stand your project up vertically as intended.

Note: For small six inch backsplash pieces, you can paint the epoxy on using a clean, tight paintbrush. Then use a heat gun or propane torch to remove the brush strokes and smooth out the epoxy. After that you can apply your flood coat to the main surface.