Gas and Oil Home Heating Furnaces - Is it really safe to use 409 on outside coils

I recently had my unit cleaned and its only been 3 days, The musty smell I had is now back. Its driving me crazy. The guy came here, sprayed coil cleaner and thats pretty much all he did. I feel like I was cheated on the cleaning of my unit. Can anyone help? I would rather do it myself than to have the hassle of trying to find a good repair person. People are too money hungry and I just can't afford it. Where would I get the coil cleaner to do it myself?
Thanks

A musty smell could be a builup of gunk on the cooling coil and the drain pan or it could be builup on the inside of the duct. I generally pull cooling coils out of the unit and take them ouside for a thorough cleaning. The bad news is, it's expensive. 409 is safe to use on the coils. You could try Graingers or Johnstone Supply for the commercial grade cleaners. but I must warn you to be extremely careful. You need heavy rubber gloves, eye protection and a well ventilated work area. In addition, the coil must be THOROUGHLY flushed with A LOT of water. This is messy if done in place inside the house which why I pull the coils. If you decide to try the commercial cleaner, get an alkaline based, foaming action product.

Forty years ago there were no coil cleaners, we used liquid laundry detergent. You can't believe how nasty-dirty a coil can be in a bowling alley, restaurant or bar back in the days of unrestricted smoking. We used a STRONG solution for cleaning and I am not aware of any damage for a decade or two preceding the introduction of coil cleaners.

The smell is from the mold or mildew growing in the dirt embedded inside of the coil. This dirt needs to be removed from the coil and drain pan.

At the bottom of the forum page is a search option. Enter "coil inspection cleaning", press "go" and read the 2 threads that it finds. This will give you guidance on the project.

The product I used was called ACTI-CLEAN and was bought at Graingers. It is safer than the alkyline produts and you DO NOT rinse it off. You spray it on with the unit running and it sucks the product through the coil. It worked great for mine. If one is extremely clogged it may not work, but for my musty odor problem it did work.