While I don't have any experience with painting over hot glue, it may have to do with the exact type of spray paint. Acrylic, enamels, etc...they all behave differently on different materials.
Other than that, I suggest trying styrene bars/strips. They can be heat formed, come in all sorts of shapes and sizes, and if you have a model railroad shop nearby that sells kits, they may carry them--kit bashers LOVE these things--and if not, they may be able to order them. Most of the details on that pauldron can be done in half-round and/or angle beam strips, and (not knowing your skill with a glue gun) will likely come out looking more uniform.
The strips are about $4 for a pack of 4-8, depending on the shape and dimensions. It's how this hilt's details
were done (NOT my work, credit to Turi @ GS props out of Houston, TX)
As for actually building the pauldron...best I can think of is to make a positive mold from some thin plywood and insulation foam, seal it with acrylic, bondo over it to make it rigid, then form plastic/craft foam over it with a heat gun. The form should rouglhy be your size--you can correct the fit using spray-in insulation foam and carving it back out later, but closer now is good.
I'd suggest looking up Volpin Props' build of Claire's armor (Claymore), Harrison uses the technique on it (only difference in your case would be how you form the plastic, not everyone has a vacforming machine in their garage).