Remove Mildew Stains And Hardened Tree Sap From An Aluminum Trailer's Exterior

If mildew and tree sap have caused black streaks and hardened residue to appear on the sides and back of the aluminum trailer that you use to store landscaping materials that you tow behind your truck, clean the trailer's exterior with the following steps. Once the exterior has been restored to its original condition, protect it from additional staining in the future.


  • water hose
  • bucket of warm water mixed with detergent
  • mixing stick
  • stiff-bristled scrub brush with extension handle
  • bleach
  • sponge
  • putty knife
  • utility knife
  • mineral spirits
  • soft cloths

Remove The Mildew Stains

Rinse off the trailer's exterior with a strong stream of water to assist with loosening and removing loose dirt. Afterwards, make a cleaning solution by filling a bucket with warm water and adding a small amount of industrial-strength detergent to it. Stir the contents in the bucket until they are thoroughly mixed.

Use a scrub brush that has a long handle and stiff bristles to apply the cleaning solution to the mildew stains. Push against the brush's handle as you move it back and forth over the mildew stains. Stubborn stains may require a stronger cleaning solution. A small amount of bleach that is added to the soapy water will assist with eliminating them. 

Protect The Trailer From Tree Sap

Soak a sponge in a bucket of water and press it against the hardened tree sap. The water will help soften each spot after several minutes, making it easier for you to eliminate them. Use the edge of a putty knife or a utility knife to lift each hardened section from the trailer's exterior. While completing this step, be very careful so that the tip of either tool does not press against the aluminum and scratch it.

Once most of the sap has been eliminated, pour a small amount of mineral spirits on a cloth. Rub any residue that remains on the trailer's exterior with the cloth. Afterwards, rinse the trailer's exterior off with water.

Now that the exterior of the trailer is clean, protect it from additional staining. If you usually store your trailer outdoors, invest in a cover so that the aluminum isn't subjected to moisture or tree sap. Otherwise, store the trailer in a garage in order to keep it protected. If you are unable to do so, spraying the trailer's exterior off regularly and removing any residue from its surface will prevent large stains from forming. Contact a business, such as Hillsboro Industries for more information.