I'll try my hand at spray-transfer for the first time Monday, Oct. 31. Using .030" ER70S-2, my chart says to use 98% argon, 2% oxygen. I only have 100% argon available. Would this cause any problem? The chart further goes to dictate 100% argon for larger wires, but I think I'll stick with the .030" 'cause the duty cycle on my Miller 251 may be a little low for the higher amps required. I'll be unavailable this weekend if you've any questions, so here is all the info I know currently:

3" thick plate.
.030" ER70S-2 wire.
Filling a 3/4"X3/4"X6" slot that will be machined flush.
Gonna' cap it w/ ER70S-2 GTAW for a mold surface (using MIG to fill the void quickly, TIG to eliminate possibility of porosity near the surface.)
Chart says 200 amp, I forgot the volts but I'll check Monday and will have to play with it to get a good wire speed.
I've tried globular transfer a few times using CO2/argon mix with mixed results. Sometimes porosity, sometimes cracking.
Plate material; 1045.
Pre/post heat; 250F./400F.

Perhaps the spray transfer will work better? I can't see me filling in this whole void W/ GTAW. Numerous attempts have been made to use a piece of steel to fill in the void and weld the joint (GMAW, GTAW and SMAW processes at different times). They all resulted in cracking around the insert during use of the mold (runs @ 250-300F. during use and frequently cooled quickly), so this is to see if filling the void entirely with weld will eliminate the crack at the seam, as the crack seems to come from the bottom of the insert, where you can't reach w/ the welder. YEEeesh! Perhaps I've put a lot more out there than just a question of the gas, but any comments are welcome.

I don't think the gas will be a problem, but if anyone is more familiar, please let me know. Thanks.