Project with Mod podge ,vase and shells

You could take a standard vase, glue seashells all over it in whatever pattern you like. Use a good craft glue like E6000 or even hot glue to attach the shells. Then use Mod Podge (a decoupage liquid) to completely paint over the entire project to seal it and give it a sheen.

You can buy Mod Podge in spray form which is much easier to use to seal uneven surfaces.

One reason for using shells is to have the "shell" look. They are a natural product and there are many grottos and shell "houses" in Europe and elsewhere. I would use hot glue to fasten the shells, do not coat them with anything.