So in my NY apartment I took down our two ac units to clean them. The front and rear fins and ac coils were very easy to get clean using a little soap and the garden hose.

However, on both units (different brands), on the rear set of coils, not the very exterior, but just behind the fan, there is sludge buildup that I cannot for the life of me remove. It is just behind the fan stuck on the fins.

I have tried spraying it out from the inside out. I have tried spraying it inward. I have used ac coil cleaning spray foam and hot garden hose water. It just will not come off. It moves around a bit but still sticks to the fins.

I ended up purchasing two new units and would like some advice on how to avoid that buildup in the first place, as well as a better way to clean it, if I did it wrong.

Of course I know now to keep the mesh filter very clean. I plan on cleaning that every couple days.

What about using a breathable AC cover in the winter? Will that help much in preventing sludge buildup on the fins back there? Does much dust and dirt accumulate in the unit during NON use?

When I clean the unit, is it best to use compressed air to blow it out first in the areas that I cannot get to with the vacuum? I wondered if I should blow it out first and then wash it. Because I cannot get to the coils right behind the fan with the vacuum or even my hands.

Any thoughts would be appreciated.