Sugaring an issue for mild steel tubing?

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I read that if I were to weld, say, two lengths of stainless tubing together,I would need to fill the inside with argon or some magic "solar paste" to preventthe inside weld joints from "sugaring" since the shielding gas from the electrodeis not going to reach it.  Is this an issue for mild steel as well?
Reply:No need for "solar flux" on mild steel.- If you can jump across it you can weld it!  - anonymous old boilermaker
Reply:Mild steel not usually, I have however heard of backpurging being used when tig welding in the root pass on pressure pipe!Stainless steel for something like the food or petroleum industry, backpurging or the use of solar flux is a must when welding stainless pipe. The "sugaring" is called carbide precipitation and (now if I remember this correctly it has been a while since.....) this is when the ouside air (oxygen) reacts with the chromium in the stainless steel. This carbide precip. has a similar effect on stainless as welding mild steel without proper shielding gas. If I remember my welding instructor correctly it can have a corrosive effect on the welded area. In short it looks bad and negates the effect of using stainless steels for corrosion resistance.