If I had to take a guess, Iron Sand is most likely weak to Fire Release because of its melting point, a normal candlelight flame temperature is usually around the mark of 1,400 °C and Iron melts at 1,538°C. Keeping in mind, that this is just of a small candlelight, techniques in the narutoverse that vary in strength and size will obviously exceed that temperature of 1,400 °C and will in fact melt Iron.
This logic is fine but not always applicable, since wood and paper are both neutral to fire as well. The melting point standards are very useful to keep in mind though when creating custom elements.
Since molten iron isn't magnetic, you can't really control it and thus your usage of it fails?
This is very true though but since neither paper nor wood would melt by great fire techniques i don't see iron sand doing that anytime soon. Infact, it could even be seen as a strength towards fire.
Wind is neutral to Iron Sand probably because its able to blow it away but Iron Sand is heavy so its harder to move around.
True, wind can be used in conjunction with iron sand techniques(trying to do such a thing with customs >_>) if used right. Magnetism is consisted of the wind and earth release so in reality it should be strong towards wind since it contains wind and earth. However, like you said, strong blasts of wind are neutral to strong magnetic forces that controls the iron sand.
Lighting is neutral to Iron Sand because it can disrupt the magnetic fields controlling the sand but can't really destroy or damage the sand.
Lightning chakra can disrupt magnetic fields of equal rank as the lightning, but when seen metals and metallic customs almost all of them are weak towards lightning as it can cut through the metals. Most metals are strong electric conductors and thus allow currents to travel through them easily.
Water is probably weak to Iron Sand, since the sand is a component of earth and earth is naturally stronger than water, Iron sand could be too.
Water is strong towards regular sand, but that's because regular sand turns into mud when soaked with the water and thus becomes uncontrollable. Iron sand is heavier than regular sand and is controlled through magnetism rather than how Gaara's sand is controlled.
And its neutral to Earth?
Yes. Iron sand erupts the earth so in short terms, it should be strong towards earth but seen as neutral anyway.
Iron Sand was created by the 3rd Kazekage after being inspired from watching Shukaku, he imitated how the bijuu controlled its sand via his Jiton KG and used hisiron sand to overpower the bijuu's sand and defeat it.
No. The third kazekage studied the jinchuuriki of the Shukaku who could use the magnetism abilities. He never defeated the Shukaku nor the jinchuuriki but he used his imagination and started to be able to control iron powder through his studies of the Shukaku's jinchuuriki, who at that time is unknown. Could've been Bunkpuku but yeah. Who knows xd.
Why was Iron Sand so effective? Probably due to its weight when compared to regular sand. It's probably how Sunagakure was able to control and seal the bijuu for so long in its village since the 3rd was able to control it every time it went lose or the jinchuriki lost control.
Was regular sand release used before iron sand? This is not proved anywhere though so i'll ignore that statement >_>. What is known is that the third kazekage was more legendary than Rasa(the fourth) who controlled an even heavier metal, Gold. However, why the third was still more famous is probably because gold is harder to acquire than iron and could thus be gathered in greater quantity and become more deadly. Just a thought though. It's only proven that Rasa compressed the Shukaku's rampage when Gaara was the jinchuuriki. Not the previous one's, so that statement is not provable.
Any other questions or are we ready for some jutsus?
