TBS has been on site since September 1 last year and was on schedule to make the finish date of November-December 2016.
"We're about halfway in but working in this environment tests your resolve a bit," Mr Comrie said.
He's in charge of about 30 employees and the work goes on seven days a week. The company's experienced in this sort of work but for this project it has employed seven locals.
"They're part of our team and they're thriving here," he said.
The site is at the mercy of the elements and that's why 200 tonnes of scaffolding suspended off the viaduct is vital in creating a safe and comfortable working environment.
"When it rains here " which it does a lot " that tends to throw our schedule but we've just got to manage it. That said, we're doing okay. We haven't had too much snow over winter," he said.
This job has demanded some unique solutions to create the ideal working environment. As well as recovering every smidgen of lead paint blasted off the steelwork, hundreds of rivets have been removed and replaced with new nuts and bolts, which are binding 52 tonnes of new steel plates to the superstructure. Workers will eventually hand-tighten more than 10,000 nuts and bolts to give the viaduct added rigidity. And steel rods are also being used to strengthen the structure.
Opus is responsible for the design of the strengthening on the viaduct and a staff member from its Wanganui office is responsible for auditing that work.
"It's essentially giving the viaduct a whole new lease of life and it means it will be able to handle heavier axle loads anticipated in the next 50 years," Mr Comrie said.
The hub of the contract though is the base just south of the viaduct and off SH4. It's home to the massive generators and compressors that are powering up the sandblasters and vacuum system needed on the job.
TBS Farnsworth will use at least 250 tonnes of garnet on this job. Garnet, a crushed crystal, is fired at the steelwork under massive pressure to remove the paint.
"It reaches the workface at supersonic speed so it means there's a lot of spent garnet, paint and dust within the contained work areas. We use a 150 horsepower vacuum system to remove the waste from the work site and bring it back to the hub," Mr Comrie said.
The garnet is recycled and used a few times before its work is done.
"It's extremely difficult to shift this volume of material to and from the work face so we've done really well to come up with a system that works effectively," he said.
Health and safety and waste management were key to the operation. Simple housekeeping is a primary concern because of the location being worked in.
While work carries on, train schedules on the Main Trunk line haven't stopped but again staff safety is paramount. Some days between five and eight trains cross the viaduct and, while workers under the structure can keep on working, every train is brought to a stop to make sure workers are clear of the line.
TBS Farnsworth has adopted a KiwiRail safety system using padlocks which the workers lock to a board. Once the train pulls up, workers take their locks off to clear the area. The train doesn't move over the viaduct until every last lock has been removed and every worker accounted for.
The blasting, strengthening and repainting of the top of the viaduct is almost done and then the focus will shift to the five steel piers, the main supports for the bridge.
"Our work goes from the top down because you want to blast over dirty stuff rather than a newly painted area," Mr Comrie said.
Much of the work is being carried out with workers cocooned inside a double layer of plastic wrapped around the viaduct. At times the wind chill in the area has been down to -14C but large heaters let the crew maintain ideal temperatures for the paint to properly cure.
"One of the big challenges has been keeping the water out. When it rains here about 200 days of the year, that's an issue. The paint we use is an epoxy cure and it's temperature-dependant, so we have to control the environment on the structure as much as we can. There can't be any moisture anywhere."
There's movement as trains pass overhead so the contractor has laid a butynol-type product under the track to keep rain out.
"We've had our moments, as you do when it can rain for three days solid and the wind's bringing the rain in horizontally," he said.
While machines are used when possible, the painting remains highly labour intensive with brushes used to ensure proper paint thicknesses on rivet and bolt heads and into every nook and cranny.
For the painters, it means using hand-held lights to illuminate the work area before applying the paint with nothing bigger than an artist's brush. An inspector is regularly on site to check and recheck.
"What we're doing is everything possible to ensure there's no corrosion again. This coating is good for 50 years," Mr Comrie said.
Morgan McLean, the coating supervisors, said they were applying a three-coat system, including two coats of epoxy, starting with a primer.
"If we don't get that primer on quickly the steel work starts to bloom or flash rust. Once we've got a whole section done then we'll come back through to put on the two coats of epoxy before we're ready for the top coating.
"The amount of work going into this is huge. It's the small things but nothing gets by the paint inspector," Mr McLean said.
About 15,000 litres of paint will eventually coat the viaduct.
Scaffolding plays a major role in the job and a total of 370 tonnes of scaffolding will be erected during the life of the contract. The most spectacular is currently being built up the steel piers. Once that's in place work will start cleaning and repainting them.
"Everything on these scaffolds has been doubled up for obvious reasons - for sheer strength and loading. And everything has to be controlled for safety reasons," Mr Comrie said.
TBS Farnsworth has been on site for a year but the environment remains in pristine condition. As well, the project has brought advantages to the local community, local businesses and employment.
The environment is getting a big boost from the KiwiRail-funded trapping programme in the Makatote valley that has seen more than 300 rats, 81 stoats and two ferrets destroyed to date, an outcome Mr Comrie said must have a "very positive" influence on the whio and kiwi populations.
"We're very proud of what we're doing here. There's always something new to be worked out."
But he said taking the last of the wraps off the top of the viaduct to reveal the new paint job would be a "defining moment for us on this contract".
