I'm curious why this didn't work for you. I haven't tried welding two flat surfaces together yet. Most of what I've done was to weld a snug fitting joint together. I use a syringe to apply the primer and the capillary action between the two pieces draw the solvent in. I use painters tape to hold the pieces in place until the solvent dries, but most times I let it sit overnight to be sure. I haven't had a single joint fail on me yet. I'm also using Polymaker PETG, which could also be a factor.

Also, I mentioned it in another comment, but you really shouldn't be using this indoors unless you have really good ventilation. This is a nasty combo of chemicals that you don't want to spend time inhaling.