Help with soldering..

OilCityPickups

Pretty much the only time I use flux paste in pickup building is in putting covers on humbuckers, and soldering earth wires to steel Tele baseplates. 
The secret of soldering starts with a big enough iron and tip for the job ... the iron should be very hot, and the tip large enough to transfer a lot of heat very quickly without the heat having time to conduct everywhere else and damage components. 
My standard pickup making Iron is a 60w temperature controlled solder station ... pretty much always used 'flat out'. For fitting covers I use a 150w iron with a 1/2 inch tip. The 60w station never has any issues soldering to the back of pots with no 'roughing up' and I've never heat damaged one. 

Roland

Much as I like to use resin cored solder there are times when things do need cleaning first. I use flux paste when the back of a volume pot won’t clean up with emery paper. It worries me every time because of what the acidic residue might do over the years. It’s not easy to wash it off pots.

There’s another use of flux, which is controlling solder flow. Yesterday I repaired the nozzle of an old oil can. It needed petrol to flush out the oil, wire brushing to get the paint off, followed by flux paste along the join line to make sure the solder ran into the right place. Resin cored solder would have formed a band the width of the iron tip.

You can tell how infrequently I use flux because I bought the tin over 50 years ago. 

hywelg

Laco flux for cleaning Fluxite for building up a joint. Tallow for flowing solder. All highly undesirable for electronics work however unless you clean up well afterwards.