Some of the hot glued shells have fallen off the frame. I didn’t sand the frame since it has the color/finish I like and I don’t want to “cover” the frame entirely. I used Tacky craft glue but it took too long to dry. Crazy glue didn’t work because not all shells have a smooth surface to attach to the smooth surface on the frame. Gorilla glue didn’t work because I had to apply water to one surface and then “clamp” it - impossible to do.
There are two challenges to your project as I understand it...
- The frame (and maybe even the shells) has a nice smooth finish which glues don't adhere to well.
- The frame is mounted on a wall, making clamping difficult.
Abrading the backs of the shells with a wire brush may roughen them up, allowing glue to hold onto them better. Drilling small holes through the frame finish in spots which will be covered by shells would serve the same purpose for the frames.
As for clamping, use painter's tape. After you glued a shell where you want it on the vertical frame surface, secure it with tape to give the glue time to dry. The tape may not be able to supply much pressure to the drying joint, but immobility alone can help form a strong bond.
With those challenges behind you, the choice of glues is based solely on the materials involved. I've always had luck with gorilla glue which comes in a gel version for flow-challenging uses.
Technically, I think what i'm about to suggest might be friction, rather than adhesion, but depending on how large the shells are and how hard the frame is, you could try sticking the shells onto your frame with thumbtacks. This would allow you to use a slower-acting glue for the shell/thumbtack bond, as well as allow precise placing of the tack-mounted shells once they're bonded. If there is any risk of the thumbtack being visible make sure to paint the heads with nail polish in a color to match either the shell or the frame. If you are clustering the shells, you might be able to avoid exposed tack heads by gluing them on in these prearranged clusters. When you get to mounting the shells onto the frame, I would advise you to gently "pre-poke" placement holes with a needle pushpin and then slide the thumbtacks in. Ideally you would make the hole smaller in diameter than the thumbtack pin, but if it is loose, a tiny dab of glue would work here too. If you really screwed up and made a large hole, dip a toothpick into elmers glue, stick it in the hole, break it off, then stick the thumbtack into the mushy wood of the toothpick.
I like using E6000, it works with nearly every surface unless its too slick. If the frame is lacquered and shiny, I would use just a bit of some acetone just on the spots where you are gluing, just to de-slick a little. I also agree with the painters tape to hold things in place while the glue is curing. An alternative to the E6000 that I've used and like is Aleene's Jewelry & Metal glue, its like superglue gel... so, be careful where your fingers are.
I have made many seashell covered frames over many years so I completely understand the problem you are facing by having to glue the shells on a vertical surface. I am confused though because you say that you like the finish on the frame-I usually make sure that the shells completely cover the frame, so it hardly matters what the frame looks like. And if there is any frame showing, it would be minimal. So acetone and sandpaper would really help!
With that in mind, I would definitely opt for E6000 and maybe some very sticky clay. The clay would just be for positioning, the E6000 would be for fixing. And, perhaps, you could even make a mix of the two, that gravity would not pull down. It’s worth a try and not very expensive! Good luck!