My tank looked about the same as that. The problem I had was that the rust had gone deep into the seams on the underside of the tank which caused pin holes through this seam.
I only found this out when the tank had been blasted and sprayed
Luckily I had a good mate with a welder and brazing rods who sorted it for me.
I then sealed it with POR 15, a three stage process and looks to be good so far. I bought it from
www.frost.co.uk