GALVANISING AND PAINTING PREPARATION
Depending on the type of finish you are planning for your trailer, there are a couple of things that need to be done prior to either. If you are planning on galvanising your trailer, please read this first - Galvanising and painting here - Painting
If you are painting your trailer, you will need to drill drain holes in the tailgates as shown below to allow rain water to drain from the tailgate. 
With either option, 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 box section uprights, drawbar and chassis rails, including the tailgates.
Galvanising drain holes are extremely important to ensure that your trailer is fully coated with galvanising, inside and out, and to prevent your trailer deforming when in the galvanising bath.
The cross members ideally should have been pre-drilled prior to the chassis being welded, but if not now is the time to do this. Two holes at least 7mm in diameter (10mm holes are ideal for the cable side of the trailer to allow the cable to pass through) must be drilled on one side of the vertical face of the cross members at each end. One hole needs to be drilled on the underside of the cross member at each end.
Wherever a piece of hollow section (SHS or RHS) is blocked, it requires drilling. The tie rails along the sides require a hole drilled through both sides at the junction with the end uprights.
The side panel requires holes drilled on the top where it meets the end and centre uprights (see pictures below)
Drill holes in the light channel as shown above to prevent galvanising from building up around this area.
Holes also need to be drilled in the top of the centre uprights, just under the tie rail. At the bottom of the centre uprights, there needs to be a partial opening at least the size of a 7mm diameter hole or bigger. If not drill the underside of the guard gusset into the upright opening.
Lastly the deck joiner sections need to be drilled with two holes on the underside and one hole on the side passing all the way through. This needs to be done on both ends of each section.
Some galvanisers require the guard to be drilled in a couple of places where it meets the panel - Check with them prior to drilling even more holes in your trailer.
Drawbar Drilling
Determine where you require your chain to be bolted to the drawbar and drill a hole to suit your mounting bolt. A good position is approximately 50mm back from the front of the drawbar and at least 1/2 a chain link length up from the underside. If you are galvanising, drill the hole oversize by about 1mm to allow for galvanising build up.
You will need to have your light cable and rubber grommet to measure up for the light cable exit hole on top of the drawbar. Position this behind the coupling and centrally on the top of the drawbar. Again drill the hole slightly oversize to allow for the galvanising build up.
You can pre-drill for your lights and number plate if you have these to hand. Most new lights will come with a drilling template or guide. See the following page for more details