Front derailleur won't spring back to lower gears The front derailleur on my mountainbike sometimes gets stuck in high gears. When this happens and I want to shift down I can nudge it a bit with my foot and it will spring back to the lower gear.

I think the spring is still strong enough because just a small nudge is enough. I haven't been fiddling with the two adjuster screws, and -as I said- if I nudge the derailleur after shifting it jumps to the correct position. The issue seems to be the (admittedly pretty large) amount of gunk that has gotten stuck in the derailleur over the years. I tried applying some WD-40 to the moving parts but two small applications have not really helped remove the gunk. Is this still the way to go or is there a better way to get the gunk out without completely removing the derailleur (which I -for now- am too lazy to do).

This question deals with a similar problem, but it hasn't moved beyond the diagnosis part (which I have already done I think).

[update:]

I tried liberally applying WD-40 a few times over the course of a week, including some (aided by hand on the derailleur) shifting after each application. This mostly removed the issue when shifting from 3 to 2, but 2 to 1 remained. Today I went to my local car-cleaning station and applied a high-pressure hose to the derailleur. This removed all the visible gunk. I applied some oil,  now shifting is as smooth as can be given the state of my chain and chainrings.
If you believe you have diagnosed the problem then a clean and a service should be all that is required. You don't have to remove the FD.

WD-40 will not displace mud effectively. I would: 


use a hose with a decent amount of pressure to remove the gunk, wait for it to dry. (be careful not to spray around the bottom bracket)
apply WD-40 to any moving parts on the derailleur but focus on the pivot points to remove old lube. 
once the WD-40 has evaporated apply some decent chain lube to the pivots.


If this doesn't fix the problem and the cable is moving freely the derailleur may be bent or the spring has lost the required tension.