Is this soldering iron too weak?

Melt some solder on the tip of your iron... If you have solder on it the cold solder will melt much quicker...:BEER

 

That iron should be fine.
As mentioned, tinting the tip is a must.
Also, I find scuffing the back of the pot helps alot with getting solder to stick.

 

Could also be you need a new tip. 25w should be enough for soldering on guitars - amps may need a little more power. Good advice above too.

Dana O.

 

25 watts should be sufficient.

Plug your iron in for 10 or 15 minutes first to get it hot enough.

Tin your tip. Wipe on slightly damp sponge to clean after every joint.

Tin the wires you want to connect.

Melt a small amount of solder on the tip of the iron before making a joint.

For a pot it helps to clean/degrease or lightly sand the surface to increase adhesion. Also use some flux (just wipe a dab on there).

As the world's worst solderer, after I learned these few points it became much easier.