Trailer build Ep14 - Hot dip galvanising

Hot dip galvanizing is the best anti-corrosion treatment for steel trailers that must go into salt water. The Zinc bonds to the steel in a thick, high abrasion coating which also has electrolytic protection. even if some coating is chipped off, the surrounding zinc will protect the exposed steel patch. Trailer life should be 25 years or more.

Most of the work for hot dip galvanizing is in the preparation stage.

The galvanizing plant needs the metal to be completely bare.  Molten zinc wont stick to paint or oil. They have an acid cleaning bath which will remove light rust, galvanizing and mill scale. Also a flux bath to help the zinc bond with the steel at a molecular level. The coasting will not peel away like paint can.

Pre-galvanized steel will consume more acid, so some plants charge a bit for this.

The trailer parts are lowered into a large vat of molten zinc, usually on an angle of 30-45 degrees. There must be holes drilled or cut in corners so air trapped inside can escape and allow the molten zinc to reach every corner (airpockets also "explode" hot zinc out) also the lower holes allow the molten zinc inside to drain out.

I drilled 8-12 mm holes in every corner i could think of. Some parts can be V-notched with a grinder before welding together. Remember the part will hang at an angle. Even the open chanel beams had to have drill holes, that the foreman got me to drill out once at the plant (so always take drill bits and a cordless drill to the plant)

Removing paint etc -

The ultimate quick way would be with a huge commercial sandblaster unit but this would be expensive and messy if done on site/ or involve transport to a sandblasters factory.

Angle grinding proved to be the fastest cheapest method for open areas (after trying wet sandblasting and chemical paint stripping much slower and more costly).

Murphy's law - My old Makita 5" angle grinder decided to stop just prior to the job. So had to order in new carbon brushes....

Older makita 5" 910W angle grinder

Couldn't wait, so bought a new 5" angle grinder to keep it rolling.  It was cheaper, less powerful but allot lighter and appears to be brushless but didn't say so.

Makita 5" 710W angle grinder

Having the two different weight and power grinders was useful. The heavier grunt stuff done with the 910W one and lighter touch up work (like chamfers etc) with the 710W one, which is more like an old 4" grinder, so wearing grinding discs go onto this one too.

Alternatively a "Tercoo" rotary blaster could have been used. I belatedly ordered one in from Brisbane but the trailer job was done before it arrived.

Tercoo 3-disc rotating blaster on drill (usually cord drill for more power)

The Tercoo is a rotary mechanical sandblaster. Tungsten tips are embedded in rubber discs and "hammer" the paint and rust off steel. It was invented in Holland and is used more in Europe, so is not an off the shelf item here. It would have been good to try the Tercoo on this project, but it must be used on flat surfaces and cant hit bumps or sharp corners otherwise the tugsten tips might tear out. Here is a short video clip of the Tercoo being used to clean a yachts keel for epoxy painting. I bought my 3 wheel  "Tercoo" from McIntyre Equipment and it arrived a few days later in the post. In any case i will want to try it out on my steel wylo2 yacht hull.

Once the angle grinders had reached all the reachable bits, paint etc was still present in difficult to reach corners. I used automotive paint stripper followed by a wet sand blast for these tough spots.

The wet sandblaster unit is a Karacher K2 1600psi water blaster. It has a sandblasting attachment - one end goes in a bag of dry graded silica sand (or more expensive garnet) and a venturi effect sucks the sand out and its merged with the water jet near the nozzle.

Early wet-sandblaster test (wrong kind of sand used but it worked)

Like trying anything new It took allot of experimenting with sand at first..i tried Bunnings paving sand , it clogged the nozzle because it wasn't dry or graded. They had garnet but that was very expensive. So i went to the sandblaster supplier Burwell technologies in Garbutt, they had the correct kiln-dried crushed glass medium for my wet sandblaster (critically had to be 0.2-0.8 mm size for the Karacher) I bought 3 bags at $12 each and used 2 bags up.

Sandblasting medium bag - crushed glass (silica)

The automotive paint stripper was "poly" paint stripper - for enamel or car paint. (Bought the house paint stripper first - almost useless on enamel)

Automotive enamel paint stripper - 1 litre, solvent base with Dichloromethane 30-60%

{Sorry these are on their side, cant get them rotated..! old programs or old me!}

Small parts ready for wet sandblasting corners etc

Wet sandblasting is still a messy wet business. So did it outside away from things. wore good personal protective equipment (known as "PPE"in Australia). Offshore sailing waterproof leggings, steel capped gumboots, welding apron, welding skull cap (rip brim off old baseball cap) and a full face shield.

The wet sandblaster feed in at the nozzle also clogged up fairly regularly, which needed pulling it apart and cleaning it out. So its not a large scale machine - just for small jobs.

One of the last jobs was to punch the identification markings into the trailer frame. VIN number, Tare 200 kg and ATM 1200kg etc from the Queensland Transport registration document. added first year built 1975,  2019 for this re-build, name of boat etc.

Draw bar stripped down 

 Once the parts were stripped bare and ready i phoned the hot dip galvanising plant,  i had already  got advice and a quote from the nearest one in the Bohle about 40km away (Used to be Goerdhardts, and been running for the last 50 or more years, now Valmont Coatings) but it had suddenly closed down, out of business without warning almost overnight !! (A sign of the poor state of our industrial/ mining economy) + (even though allot of huge solar farms were being built most of the engineered parts were shipped in and not built locally..i heard)

So contacted APG Australian Professional Galvanisers (good video of process) in Stuart, south of Townville about 70kms away, they could do it.

Loaded the trailer frame onto my utes roof racks with the help of the 1/2 ton chain block, a mini crane. The main frame must weigh 120 kg's or so. (i can just roll it over sideways unassisted, but lifting up high..) The Ute felt quite top-heavy, kind of wobbly and rolly, so took it easy at 85-90km/hr and went via the 60km/hr streets through Townsville. The old ute didn't have much evasive action in her, so kept large distances from other cars when possible.

Trailer frame and parts en-route to galvanizing plant.

 At APG - It looked like a much larger operation to the visitor than the Bohle plant. They were dipping some seriously big structures in there, big trucks and forklifts going everywhere.  My job and ute looked like a kids toys by comparison but happy they could do it among the big buck work. Wayne the foreman was really helpful, and got me to drill more holes into it which took 30 minutes.

About a week or so later on Tuesday the call came in , it was done.! At a fair price too (the going rate in tsv was about A$2.50-$3 per kg) So picked it up and drove back to the boatyard via the outer ring road during quieter midday traffic. It felt like alot easier drive this time.

The galvanized trailer arrives at Boat yard

A few dings in the 10mm rod to bang out

 While unloading i tried an alternative main frame loading method. Direct on the tray with tail hanging out. Although untested i reckon the center of gravity would be so much lower and the ute far more stable like this.

Alternative frame carry position

Lifting the frame off i found and photographed all the balance points

Unloading with chain block

Frame lift balance point when upside-down

Frame lift balance point on side

Frame balance point upright

Balance point is 4 1/2" forward of frame 4. Note galvanizing hole.

Ready to roll wheels underneath

The other thing hot dip galvanizing won't ever stick to are pinprick holes slag in welds. I painted cold-galvanizing Zn paint on these imperfections while the frame was hanging from the chain block.

I persevered through this whole fabrication and galvanizing process as a pre-cursor to building my wylo2 yacht fittings - these are also hand fabricated in mild steel and will get the hot dip galvanizing treatment too.

Its given me insights into trailer design and building too. Hope to draw up plans for this "Teria Mk2" trailer and an even better "Teria Mk3" trailer using info i've gleaned along the way. The Mk 3 will be easier and faster to build (would use 3x2" RHS V-cross beams and 4x2" side beams and draw bar without bracing) It could be used by others who want to re-build their own Investigator 563 trailer. As a non-tradie and learn-as-you-go amateur with a growing family and work, the Mk2 took allot longer than first envisioned!

If you're cashed up and working allot its probably better to pay for a professionally made trailer. They would use black steel (no paint, minimal cleanup prep) know where to include the limber holes before welding, do clean welds and have it ready to galvanize before hardly any surface rust even set in. They probably have easy access to proper sandblast equipment which would make quick work of surface preparation if required.