I know many of you have said soap, Guinness, warming the whistle, etc are your tricks for stopping the build up of moisture in the fipple. Is there a more permanent solution that is easy to apply and not detrimental to the whistle or the sound?
Condensation is to be expected when you mix warm breath and cooler air temperatures. Aluminum is a terrific conductor of heat. It also cools quickly. So cold conditions, warm breath, solid aluminum whistle head - ba-da-bing, ba-da -bang - moisture build-up! Or is that did-lee-dee, dee-da-die?
Anyway, different aluminum (or other metal) whistles will bog down differently. Windway height can be a contributing factor. The finish of the windway components can contribute. You did not state what make you are playing and having the issue with. Some whistles are worth the effort, some are not, IMO.
Moving the moisture along works to a degree on most whistles. Some blow the condensation out, some suck it out, some shake the whistle head towards the banjo player. Pick one. Be polite if possible.
Warming the head works the best. Breath into it before playing while covering the window. Place the whistle in a towel wrap or heating pad while not playing it. Ever seen those socks with the electric warmers that they sell for outdoorsmen? Stick it under your armpit. Lots of things can help.
If that doesn’t work, get an aluminum whistle with a nice smooth delrin plug and windway - like a Burke perhaps. Go for wood. Try polymer whistles.
I ramble on.
Feadoggie
I am playing a Domnahl na Gruen low D, the windway is all aluminum and the block is a blue colored translucent acrylic. My biggest issue is that I have a hard time finishing a song before I need to clear it. I may have to look at some type of heater for it, as holding the mouth piece in my hand and under my arm and blowing through it prior to playing does not help me get through a tune. Other than that, it is a fine whistle with a wonderful soulful sound on low D and has a good controllable jump.
You can use a dew heater, which I use on the eyepieces of my telescope.
The one for 1 1/4" eyepieces works just fine. You need a controler for it.
Google for “dew heaters”. There are cheap ones and expensive ones. Pick a cheapo…
Just my 2 cts. !
Don’t ask what happens to the Bodhran players!!! 
Duponol?
I am sorry… are you asking me if my whistle has duponol in it?
I was asking if you have tried duponol instead of the other substitutes. It can be found in music stores or on line.
Use it the same way as soapy water. Coat the inside of the wind way, let it dry then play.
Duponol is a good place to start.
I have some experience with Domnahl’s (Daniel’s) whistles since I own a low D, low F and high D. They are very decent instruments. The low F is particularly nice. But like many low cost, low key instruments there are compromises. The fipple block Daniel uses is, I believe, acrylic. He uses this for artistic effect. It does not shed moisture like delrin seems to and since Daniel uses very light weight aluminum, it is difficult to keep the instrument warm. But this light weight aluminum allows very interesting overtones and in combination with the ergonomic hole placement this can be a nice affordable place to start on low whistles.
The only way I’ve found to accommodate these two issues are Duponol and “sucking back”. The recorder players on this forum will know what I mean. Duponol will last longer than soap though you do have to pay for it. I’ve since moved on to playing instruments made by others so I never got to try MTGuru’s trick of using dental floss on Daniel’s whistles. If you search this site you can find references to this technique in a few places and it sounds logical to me.
But you should still learn to “suck back” since if you’re a wet player (I’m not) you’ll always have this issue to deal with. Some of the best instruments made will tend to clog occasionally for the wet player. You can’t always blow them during a tune and I’d advise against shaking them at the Bodhran player. Some of those guys are big!
ecohawk
Thanks a bunch! I ordered some Duponol online for about $2.50. I don’t think I am a wet player, as this is the only whistle I have issues with. But I like the sound of it and the placement of the holes. I think that the Duponol is a good suggestion.
And they like to hit things.
That’s why you should consider playing a Low whistle or flute - peace through Assured Mutual Destruction 
The Duponol works great! Thanks for the suggestion
See also this old thread about Duponol, where some health concerns were expressed:
https://forums.chiffandfipple.com/t/duponol/52910/1
And if one shares these health concerns they might also want to look into how they are washing and rinsing their dishes and eating utensils each day - same stuff by and large.
Could cleaning up after the Exxon Valdez and the Deep Water Horizon oil spills have caused more harm than good? Dawn to the rescue, so to speak.
Just a thought.
Feadoggie
Hey ecohawk
I resemble that remark 
David
And you don’t taste it off of the fipple! 