I need to clean the condenser coils on my central A/C unit. The lower part of the coil is caked up with dirt and or rust. I use coil cleaner twice a season but I noticed the bottom on it didn't want to come clean. I used the foaming aerosol can you get at Home Depot. Is there something stronger I can use or do to dissolve the caked on dirt/rust on the coils before I cover it for the winter. I had the system checked the end of last summer and all was fine.