Multilayer skull projects - do you paint before or after gluing the layers?

Paint before assembly, but mask the glue joints. That delamination you got is classic wood glue on a sealed surface, it needs raw wood to bite into. What I do is paint each layer, then hit the back edges with a quick pass of sandpaper where the glue will land, just enough to knock the paint off. Glue on sanded paint will hold okay, but glue on bare wood is way more reliable. Masking tape on the glue line before painting works too if you'd rather not sand, but sanding is faster in my expirience. Wipe the dust off before gluing or you'll get little bumps between the layers. For recessed areas between layers, honestly most of the time you don't need paint in there at all. The depth of the shadow does the work. If you do want color in the gaps, spray the back layer before assembly and let the stack itself hide the messy overspray, since nobody sees the inside faces once it's glued up. On the fine details like spiderwebs and thin fence lines, your instinct is right. Spray the whole layer one color and let the cut gaps show the layer below. That's the whole point of layered designs, the contrast does the detailing for you. If a web needs to be a diferent color than its background, paint the sheet flat first, then spray or brush the whole face and the fine cuts pick up the undercolor naturally.

For anything that really needs hand detail, a cheap set of foam brushes and those tiny artist brushes meant for model painting get into spots a regular brush can't. Or do what I ended up doing, cut a few test swatches of the same plywood with the same fine lines and practice on those before committing to the good piece. Beats ruining another afternoon on the actual project.

Can second the spray trick, it saved a similar stacked pumpkin I did last fall. I shot every layer flat black first and let the cut lines do the work, and honestly nobody coud tell the webs and fence rails weren't painted separately. One thing I'd add is to hit the recessed face of the back layer with a lighter gray or off-white instead of a second dark color, the layers pop way more when there's some value difference between them. All black on black went invisible under my shop light but looked fine on the porch, so test it where you'll actually hang the thing.

One thing that saved my sanity on a stacked cat sign was drilling two tiny index holes through each layer before anything got painted. Run a toothpick or piece of wire through them during glue up and everything stays lined up while the clamps go on. Once it's dry you never see the holes. Guessing at alignment with four layers of squishy glued plywood is a good way to end up with a wonky skull that took three afternoons.

Tried the spray-everything-flat trick on a small project last month and it worked better than I expected, though I did get some paint bleeding under the tape onto the raw edges I was trying to keep clean. Ended up sanding those bits like was described above, which sort of solved itself. My question is about glue, does the yellow carpenter's wood glue matter or is plain white school glue fine for something like this that just sits on a shelf? I've only got the white stuff in the garage right now and don't want to make a hardware store trip if it doesn't matter.