Hi Derek,
My thoughts are that you need more heat for a start, as that 25 watt iron will cool very quickly when it comes in contact with a large metal surface area. Maybe try a 60 watt iron with a decent sized tip and it should improve your soldering success rate.
Big heat, short duration, is often the way to go.
I use an 80 watt iron for most of my work, including soldering whitemetal model parts, and if you are quick, good result, If you are a bit slow, molten parts! Not a problem for me, as they then go back in the melting pot and I make some new bits!
Ian.