When stain turns a milky color, it almost always indicates moisture contamination. This is usually caused by applying stain before the siding has completely dried.

Since you pressure-washed the cedar, that’s likely the cause. Pressure washing can drive water deep into the cedar. Even when the surface appears dry, there can still be moisture present inside the wood. After stain is applied and heat from the sun warms the siding, the trapped moisture will migrate to the surface and cause discoloration of the stain.

This can be a difficult problem to solve. The first thing to try is nothing - let the siding dry (hopefully) on its own by releaseing the moisture naturally. This can take a long time because the stain will slow the process.

After several months pass, you can try cleaning a small test area of the discolored siding with a nylon bristled hand brush and oxygen bleach. Oxygen bleach is available at most home improvement centers and is much safer to use than regular chlorine bleach. Don’t rub too hard - let the oxygen bleach do the work. Rinse with a garden hose and let dry thoroughly. Then try to stain the test area and see how it looks.

If this fails, you can move to either a semi-transparent or solid stain. This of course will hide the look of the wood. Painting over a previously stained surface is extremely difficult and best left to professionals.

Most manufacturers do not advertise their products as being “maintenance free” but, as low maintenance. The biggest disconnect is often the fault of the purchaser, not the manufacturer’s advertising.

You are correct in stating that these products require periodic maintenance just as any product that is exposed to the elements.

As way of example, it is unrealistic to believe that a car stored outdoors would keep its shiny paint job without ever washing it.