Painting aluminum siding is simpler than painting many wood substrates. Aluminum siding is very stable, does not swell or absorb moisture, does not allow moisture to pass directly through it. In most cases aluminum siding will outlast the best of paint jobs on a wood substrate. With aluminum simply cleaning the surface is about all the surface preparation that you will need to do. Even priming as a rule is not necessary. It is basically wash and paint.
To minimize the chance on injecting water behind the siding you will need to keep the wand moving when washing and aim away from cracks and gaps in the siding. You must avoid spraying directly into slotted vents or openings and avoid spraying anywhere near electrical outlets and light fixtures etc...
Applying the cleaning solution
There are other cleaning solutions that work well, but the one mixture I have the most experience with for cleaning aluminum siding is a Jomax and bleach mixture. Jomax is a mildew killing bleach activator manufactured by Zinsser. I apply this solution with a garden sprayer and after the appropriate dwell time, you can soft rinse it off, and follow that up with a hard blast to remove any loose chalk.
Low pressure cleaning
One good method I have found to minimize water injection behind the siding is to use low water pressure cleaning in combination with a scouring pad attached to a pole sander head. (You will need to use an adapter to use a telescoping pole with the pole sander)
Admittedly you will have to provide a bit more muscle with this method but really not a whole lot more. Dragging the weight of the wand and the high pressure hose up the ladder 20' feet into the air involves a bit of effort too, so I am not sure which really is more work - both methods of cleaning are work.
Low pressure cleaning involves using a low pressure applicator (garden sprayer) to apply the ( Jomax ) cleaning solution. Once the cleaning solution is on the siding for 5 - 10 minutes take scotch brite pad and wipe the surface to loosen the chalk. The scouring pad will also degloss the siding. The best set up for the scouring pad is to attach the pad to a pole sander head attachment. The pole sander head can be screwed onto any length telescoping pole as needed. I will usually use a 8' - 16' pole (but a 8' - 24' pole may be needed with some jobs). You must get the under side of the lap siding with the scouring pad too. Once the siding is wiped down with the scouring pad, a thorough light pressure rinse is all that is needed using a garden hose and normal water pressure from the spigot.
(Instructions for preparing aluminum siding for repainting of houses built after 1978** )
High Pressure Cleaning
The most critical step in painting aluminum siding is the cleaning. There will be paint chalk** and probably mildew to deal with. The most common method used to clean aluminum siding is with a pressure washer.
Washing with pressures anywhere from 1500 PSI to 2500 PSI will do the job. The one caution to note with pressure washing is to be careful not to inject water behind the siding through cracks and gaps in the siding and / or through slotted vent holes. You will also need to avoid electrical outlets to prevent electrical shock.
Pressuer washing aluminum siding from off a ladder
Pressure Washers
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How to Paint Aluminum Siding - Cleaning**
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** Note: If your house was built before 1978 do not wash the chalk down off the siding as it may contain lead.
The instructions detailed here are for post 1978 houses. With pre-1978 siding you will need to "lock down" the chalk using EmulsaBond rather than washing the chalk off - because the chalk may contain lead dust..
See EPA's Lead Page for more information on lead paint issues
See EmulsaBond Page
Please read lead paint warning if your house was built before 1978
