Start by ditching the argon and getting the right gas !! CO2/Argon mix.
All above advice is good. In addition, you want to cool those welds quickly - use a wet rag or compressed air.
Some holes will take more than one hit to fill. Let them cool before hitting them a second or as many times as needed.
Any hole 1/4 inch or larger needs a plug of some sort. Doesn't need to be a perfect fit, just to help fill the hole.
When grinding the welds a lot of guys use flap discs or those interchangeable discs. I find they flex too much and often remove (softer) metal around the weld and leave a bump where the weld is. I prefer a hard grinding disc to carefully grind down the weld and only the weld.