yellow plaster - sulphur?

[simpi]

I attempted some plastering for the first time yesterday. I used Wickes patching plaster on quite a small area 0.3m^2 -ish. I am pretty sure the mix was a bit too wet. Anyway, this morning, there were yellow stains in the plaster. Is this likely to be sulphur and, more importantly, is it a problem? Should I hack it off and try again?

It's a 30s house, the patch had the original wooden battens as a base.

Thanks.

[Zip 1]

This is a vey common problem, you just have to paint the yellow patches with an oil based general purpose primer before you decorate.

Let the plaster dry first of course.