Porosity With Tig On Mild Steel.

I had a small job to do the other day welding some thin brackets to thin tube.
I used argon and a thoriated tungsten.
I could not stop the porosity, it looked like foamed metal.
I played with the amps, up and down, gas,from very low to very high and nozzle diameters all to no avail.
Anyone got any more ideas?