It is sap, but otherwise known as resin/rosin – you won't remove it by dipping it (and IMO anyone that dips doors ought to be bloody shot – it often warps them if not completely ruining them). For the same reason, before painting woodwork you should always put knotting solution (shellac, iirc) on any knots (which are especially resinous) to seal them and stop the resin leeching through into the paint above and staining it
You ought to be able to remove the resin by wiping it with turps – that's *proper* turps, not 'turps substitute'. Or when the weather cools, carefully scrape it off when it sets again.
If the doors are early 1900s, and have a very noticable grain, there's a good chance it's pitch pine – that's the sort of stuff you often see in churches as various woodwork (eg pews) and looks far nicer than any scots pine stuff you get nowadays:
http://www.woodcomponents.ie/images/pitch_pine.jpg
Most of the woodwork in our last place (Edwardian, 1914-ish) was pitch pine – one door in the hall had so much resin in one patch that with the sun behind, it looked more like a patch of ambar viewed from the other side…
