Sanding for painting

I borrowed a sander earlier in the week, and this evening I decided to see how well it worked. Unfortunately it didn't work as well as I had hoped!! I did find some pretty rough paper for the sander, but the pits and paint seem pretty firm on Bones so I am not sure what to do next. I am thinking of getting my wire brush out and seeing whether I can find some even more industrial strength sandpaper to smooth it all down.

As the budget is tight I am going to go for Dulux Weathershield which has been recommended to me. Unfortunately they don't do purple or grey which is a great shame. I did think about mixing it myself but I can't guarantee I would be able to mix the same colour again. I am not sure whether I should be looking at other paint makes - such as crown, but I haven't heard others talking about them.

All advice welcome!