Hi,
I currently use the MG chemicals tin plating solution on my DIY PCBs. Unfortunately, it seems that the flux in my solder paste is eating away the tin during soldering, leaving bare copper around the soldered components. The remaining surface is also difficult to solder to, even with additional flux, so I assume it is quite oxidised. 
 The flux is synthetic no clean flux.
I have ordered some other pastes to test, but does anyone have any suggestions for other things I could try? I was thinking of perhaps a thicker layer of tin, but as I understand the immersion tin is not autocatalytic, so I’m not sure if it’s possible to get it to plate any thicker. HASL looks kind of interesting, but I can’t seem to find many resources on the levelling part in particular (which is important because I do a lot of SMD stuff).
Any help or pointers would be appreciated.