What to do about holes in rusty deck

[ThresherMan]

I started a long overdue project to clean up rust on the deck, but need some advice on dealing with a couple of places that have rusted through, hidden under paint. I tested the blown-in insulation backing and sparks from the welding will start it smoldering and burning. Is there anything I can pack or squirt into a hole on the deck that will not burn, so I can weld it shut? The locations of the holes prevent access to the underside. The holes are mostly small - ¼ to ⅜ or so in diameter. I’m using a cheap flux wire welder. Thanks for any advice you can give.

[boden36]

Can you flood the back of the area with argon or CO2 ?