First of all, you will need to completely empty the trailer you wish to clean. Remove things like horse tack, brushes, bridles and ropes.
Starting in the back portion of the trailer, power wash the floor and move all the way to the front portion of the trailer, removing all dirt and horse waste along the way. After you have washed the floor completely, wash the walls as well as the ceiling. At this point, you would need to move to the exterior of the truck and power wash it, just like you had power washed the interior.
Prepare a mixture of soap and water in a large bucket. Use this soapy mixture to wash the trailer in a manner identical to washing a car or any other vehicle. Rinse the entire trailer, from both the inside and the outside, and use a scrub brush to remove grease or horse waste stain. Allow enough time for the trailer to dry completely before moving on to the next step.
Apply metal polish to the interior of the aluminium horse trailer in 3-foot section. Buff off polish from each section before applying polish to the next section of the trailer. Use a cotton cloth or rag for this purpose. Make sure you polish both interior and exterior of the trailer.
Finally, apply a uniform coat of aluminium wax to both interior and exterior of the truck. Allow time for this wax coat to dry and then apply another coat. Wax will ensure that the finish lasts for a longer period of time.