Make sure the tip on the iron is also tinned as this makes a massive difference, maybe use a new one if possible. Good quality solder is also important.
When soldering speedo posts I have found sometimes I need to apply heat to the posts rather than the pads to get the solder flowing.
50w should be good enough although if its fairly old it may be worth investing in a new one as I know mine gets lots more use now with this RC malarkey than it ever did before.
Good luck and may the flow be with you :thumbs up:
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TLR 22,
(Its all really about the stickers tbh)
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