No bonding primer needed with most acrylics. However, depending on the condition of the siding power washing may not be sufficient prep. If it's heavily oxidized as aluminum often is, especially on the sunny sides, power washing alone may not be enough to remove all the oxidation or "chalk".
A good test is to run your hand across the siding. If your hand comes away looking 'chalky, it's oxidized. In many cases, scrubbing along with washing is required to totally remove the oxidation. In extreme cases, the oxidation of previous coatings is so extreme that's it's virtually impossible to remove it all even with scrubbing. In these cases a stabilizer can be used like Seal Krete Original, or Loxon conditioner. Products like this are made to lock down chalky surfaces and serve as a suitable primer in those situations.
Painting directly over chalk, even a little bit, is almost always a disaster as the paint bonds to the chalk instead of the aluminum and can quickly peel.