To prepare for getting the trailer galvanised, go over the trailer with a hand grinder with a flapper disc attached and clean any burrs, sharp edges, left over welding slag and spatter. Remove any paint, crayon or grease that may be on the trailer with an alcohol or solvent based cleaner.
Use a square or rat tail file to remove any burrs from inside the chassis rails, drawbar and roller components.
Galvanising drain holes are extremely important to ensure that your trailer is fully coated with protective zinc, inside and out, and to prevent your trailer deforming when in the galvanising bath.
The diagrams below outline where holes should be put. Check with your local galvaniser and check what diameter holes they require - they will charge for missing and undersize holes.