ArtMetal

I need some help getting some silver solder to adhere to sandblasted stainless steel (aluminum oxide blasting media I think). I have 304/316 stainless plates of either thickness', 0.030 or 0.060 and I'm trying to get some solid copper wire to solder to the plates for electrical connections. I started with my digital soldering iron at 750F, but didn't get the electrical 60/40 solder to stick very well. Then I bought a 230W soldering gun and it worked better, but still not good enough. I bought some Harris High Temp Black Flux and it just dries out before anything will stick. I grabbed some miuratic acid as well and tried a drop on the stainless but all it did was make the metal black and the solder roll off worse. I bought some Solder-It Silver solder paste and it doesn't work either, everything just rolls off the stainless or if it does stick I can't get it to wet in the groove for the copper wire so that I know it joins good.
I'm about to buy and try some silver solder wire that claims an acid core, but before I do I wanted to get some advice and help in the process of making these two metals stick together. All I'm trying to do is get a solid core 12AWG to solder into a .075 groove in some 16GA 304/316 stainless steel sheet metal that has been blasted to make it rougher. If my electrical load is light enough on the setup I will then change it to 22GA sheet metal with an appropriatly sized solid copper wire to it as well.
Once I get the stainless wetted once with the solder I'll be able to work with it easily after that...right? I might have to change the wires later so I'm hoping that it will be like electrical soldering and that once wetted it is much easier to deal with.