Dot and dab onto steel

Hi all, (again)

Today I tried using Gyproc drywall adhesive to board in a steel support joist. The joist is not the I shape one its a hollow rectangle shape. I made a thick mixture and put 5 fat blobs about 30cm apart along a 2.3mx 22cm section of fireproof board. It has just fallen down! (it's hanging vertically not horizontally).

I can see drops of water where the adhesive used to be on the steel, like condensation on a window.

To a newbie it looks like the adhesive has dried out ( it is to the touch) and the water has gathered between the joist and the adhesive thereby slipping off and falling down.

Clearly I have done something wrong - would adding PVA help? Or pva'ing the steel first? I did clean the steel first as it was a bit dusty.

Bit cheesed off frankly!!

Thanks in advance

A