Trailers live hard lives. Sidewalls flex on washboard roads, roofs take tree limbs and hail, and rivets loosen after thousands of miles. When the shell opens up, water follows seams, insulation wicks it across the frame, and the repair bill grows. Fixing this quickly and correctly is the difference between a clean, dry box and a long fight with corrosion, mold, and downtime. That is where on site welding services pay for themselves, especially when the damage sits on thin aluminum panels four feet off the ground in a gravel yard.
I run a portable welder set up for aluminum and stainless, with TIG and MIG on the truck, and a stocked drawer of abrasives and sealants. I keep AWS certified procedures for structural tie-ins, and I keep a mental map of the failures that tend to repeat. This guide shares how a mobile welder approaches sidewall and roof repairs on cargo, dry van, reefer, utility, and equipment trailers, and how owners can prepare their gear so a two hour call does not turn into a two day tear-down.
Aluminum solves a weight problem. A typical 53-foot dry van with aluminum panels saves hundreds of pounds compared to steel, which buys payload and fuel economy. The trade: aluminum moves with heat and work hardens. Panels expand and contract with day-night cycles, and every fastener hole becomes a stress concentrator. Over time, you see three patterns:
On the roof, add punctures from low branches and satellite bracket installs. On the sidewalls, fork tines tend to crease lower panels, and tire blowouts rip fender wells like a can opener. Aluminum welding can fix all of this, but the method matters. Choose the wrong process, contaminate the joint with road grime or steel brush particles, or chase a crack without stopping it properly, and the repair will come back.
Both processes work on aluminum. The decision lives in joint access, thickness, cosmetic needs, and wind.
TIG (GTAW) gives control. On sidewall splits around 0.040 to 0.080 inch, a TIG bead can tie the edges without adding much heat, and a steady hand can keep the puddle from keyholing through. It shines when you want a clean, narrow bead that will accept a light sand and polish. It is also forgiving for thin roof skins and delicate trim around doors and railings where overstitching would warp.
MIG (GMAW) with a spool gun or a push-pull gun puts metal down fast. On torn fender wells, cracked crossmembers, or thicker corner caps where the sidewall meets structural elements, MIG is efficient. It tolerates minor gaps and field conditions better, and it will not tie you to an extension cord for a foot pedal. The drawback is spatter if the prep is poor, and a wider heat zone that can wave a thin sheet.
High-frequency AC TIG plus a clean joint is still the gold standard for small, thin cracks. For long seams, patch plates, and overhead roof work, a well-tuned MIG machine saves time. A mobile welder should carry both, with 100 percent argon for aluminum and appropriate stainless gas when called for stainless steel welding on hinges, ramps, or railings.
If a welder arrives and strikes an arc without cleaning, expect a do-over. Aluminum holds oxide, road salt, and diesel film. That oxide melts above 3,600 degrees Fahrenheit while the base metal melts around 1,200, so if you do not break it, you end up pushing slag. My routine looks like this:
First, stabilize the trailer. Drop landing gear on firm ground, chalk wheels on both sides, and if the job is near the suspension, dump the air bags or crib it with timbers. An emergency welder will work in odd places, but moving a trailer during welding is an easy way to warp a seam or cause a secondary crack.
Second, expose the damage. Pull interior scuff plates or e-track if necessary, but avoid gutting an entire bay unless the crack runs under structure. If insulation is wet, bag it out of the way. On roof jobs, pull cap trim and sealant back at least a hand-width beyond the visible issue.
Third, degrease. Use a dedicated aluminum cleaner or acetone on a lint-free rag. Two passes. Do not reuse dirty rags. Keep open flame away from solvent.
Fourth, remove oxide and paint. Stainless wire brush or new abrasive disc, not a carbon steel brush, which embeds particles that later rust and appear as freckles under clearcoat. Work in the direction of the joint. For thin skins, a flap wheel can chew too fast, so I prefer Scotch-Brite style discs.
Fifth, stop-drill the crack. A 1/8 inch bit at the crack tip, sometimes 3/32 on thin roofs, to relieve stress and keep the crack from growing when you put heat in it.
Sixth, decide whether to back the joint. Copper backing bars help prevent burn-through on thin panels and make a cleaner root. On a roof, I often tape a copper strip under the skin if access allows, or use a handheld copper spoon if I am working solo.
A field reality: prep can consume more time than the welding itself. Owners sometimes balk when a mobile welder spends 45 minutes with a grinder and acetone before striking an arc, but that is what a permanent repair requires.
For common trailer alloys like 3003 and 5052, 4043 filler is a safe choice for TIG or MIG. It flows nicely, wets edges, and resists hot cracking. Where I see 5083 or 6061 structural members near a hinge cluster, 5356 is usually the call, thanks to higher magnesium compatibility and better as-welded strength. On roof patches where maximum corrosion resistance from sea air or road spray matters, I favor 5356 with a diligent post-weld seal.
When the panel alloy is uncertain, a quick scratch test and bead behavior during the initial tack often telegraph what you are on. If a customer provides a patch plate, I check it with a file and bend sample, and if it feels too stiff relative to the skin, I warn about different coefficients of thermal expansion that may print a seam under sun heat.
A typical job: a 24-foot enclosed trailer takes a light pole on the passenger side. The lower two panels crease, and a 6-inch tear opens between rivets. The owner needs it watertight by morning to load equipment.
I start by flattening the crease as much as practical with a dead-blow and dolly, because the less shape mismatch you leave, the less the patch will oil-can later. After cleaning and stop-drilling, I TIG stitch the tear closed, then grind the face flush with a 120-grit disc, taking care not to thin the parent metal around the weld. If the metal is thin or distorted, a patch plate works better.
For a patch, I cut a plate from 0.063 5052, round the corners, and form a shallow contour to match the wall. The overlap should be an inch to an inch and a half beyond the tear. I soft-set Clecos or temporary screws to hold the patch. For cosmetics, I prefer a weld-in patch with a tight fit, but in the field, an overlap weld gives speed and strength. I MIG the perimeter with short, staggered beads to control heat, then fill the gaps. If the trailer hauls food or sensitive goods, welds beat rivets because you avoid through-holes that invite leaks.
On the inside, I check for any interference with e-track or plywood. If needed, I add a small backing strip behind, stitch it to the skin for stiffness, and seal the edges.
After the metal cools, I seal the perimeter with a high-quality, paintable polyurethane or an aluminum-compatible butyl tape under a cover strip. Silicone is a last resort, because it contaminates future weld areas.
Roof skins are thin. Some are 0.032 inch aluminum over bows every 24 inches, with a cap at the front and rear. Holes under a branch strike usually stretch the metal, so you cannot just pull edges together. You either insert a plug patch or overlay.
When access is possible under the roof, a flush patch looks best. I trace a circle around the damage, cut to clean metal, and fit a slightly undersized circular plug with a 3/8 inch backing ring that lives under the skin. TIG is my choice here, with copper backing and a pulse setting that keeps the puddle manageable. The weld bead stands proud, then gets kissed with a Scotch-Brite wheel, not a heavy grind that thins the skin. The joint gets a coat of etch primer on the outside and a bead of sealant on both sides.
If interior access is blocked, an exterior lap patch is fine. I prefer a rounded rectangle to avoid sharp corners where cracks can start. Clean, pre-bend it to match any roof crown, and stitch weld in a skip pattern to avoid heat buildup. Where a full perimeter weld is risky on very thin skins, bond with structural adhesive rated for aluminum and add short welds as mechanical insurance between bows. Finally, replace any roof bow rivets you had to pull, and re-seal the cap. A missing dab of sealant at a lap edge is how the next leak starts.
For reefer roofs where foam insulation meets the skin, keep the torch moving. Foam burns fast and drops sticky residue. A wet fire blanket inside and a spotter make sense. Keep a 5-pound ABC extinguisher at arm’s length.
Not every sidewall job is a sheet metal patch. Impacts at the loading dock can crush the nose rail, shear welds at corner posts, or deform the header at the ramp door. These are structural repairs, and they benefit from an AWS certified approach. That does not mean bogging the job down in paperwork, but it means knowing when a cracked fillet on a corner post carries shear that a surface stitch will not hold.
On steel-framed trailers with aluminum skins, you may be transitioning from aluminum welding on the exterior to stick or flux core on inner steel posts. Switching processes mid-job requires gas management and safety awareness for fumes. On all-aluminum frames, 6061-T6 members lose strength in the heat affected zone. If you rebuild a corner with a long, continuous weld, you can create a soft zone that bends later. I favor short, staggered welds and reinforcement plates that spread load without creating large softened areas.
Hinge clusters at ramp doors deserve special attention. Stainless piano hinges bolted through aluminum skins look good, but galvanic corrosion between stainless and aluminum bites over time. Use dielectric barriers, proper washers, and if welding is needed, isolate the stainless welding and aluminum welding steps so cross-contamination does not drag ferritic particles into the aluminum joint.
Mobile welder setups have limits. Wind strips shielding gas, so a breezy yard can turn a clean MIG bead into Swiss cheese. Wind screens help, but on roofs, you are exposed. Weeks with steady afternoon winds require early starts. If you are hiring a welder, reserve a spot near a building corner or put up a temporary wind break if you need roof work.
Power matters too. TIG on aluminum likes steady amperage. Running off a generator on the truck is routine, but the generator needs enough headroom to avoid ripple that makes arc starts sticky. I keep my truck welding rig at 8,000 watts continuous with a clean inverter, and I still switch to MIG when line stability drops. Long extension cords on a flaky jobsite circuit are a recipe for frustration.
Reach is often the silent time sink. Reaching the center of a 102-inch wide roof from a ladder is unsafe and awkward. Bring a rolling scaffold or at least roof pads to distribute weight. For tall dry vans, I carry a pair of aluminum staging planks and low-profile stands. Time invested in stable footing comes back in bead quality and speed.
Not every trailer needs a show-quality seam. For a contractor’s equipment trailer, a strong weld with a neat seal beats a hidden repair that costs triple. For a food-service reefer, cleanliness and vapor-sealed joints matter more than cosmetics. For a horse trailer, sharp edges and heat near adhesives or insulation need extra care.
I ask three questions before I light up. What is the function at this location, what does it carry, and how long does the owner plan to keep it? A temporary field stitch to get a race trailer through a weekend is different from a full panel replacement on an industrial fleet unit that sees daily loading dock abuse. The right answer spends money where it buys reliability and safety.
A welded repair is not finished until it is sealed. Aluminum does not rust like steel, but it does oxidize, and capillary leaks creep under paint. For exterior seams, use polyurethane or hybrid sealants that bond to aluminum and remain flexible. On roof laps, butyl tape under trim plus a top bead at the edges works. Over old silicone, you often need to remove it entirely, since new sealant will not bond.
Paint around aluminum welds needs attention. If the trailer has a baked enamel finish, a field paint match hides the repair well enough, but do not put thick filler over thin metal near a roof patch. It will crack with expansion. A light skim to knock down a low spot is fine.
Where stainless hardware meets aluminum skin, add nylon washers or a barrier tape. On wrought iron fencing or gates transported on the trailer, avoid stacking raw steel against aluminum walls. Road moisture and time make a battery.
Customers sometimes ask what is on the truck beyond a machine and gas. For aluminum repairs in the field, I keep a few standards that save jobs:
That is one list. It stays short on purpose. The larger point is that on site welding services need to be complete. A second trip because the welder did not have a copper spoon costs more than the tool.
Three scenarios make sidewall and roof work tricky. First, ultra-thin skins under 0.040 inch. You will want TIG with pulse and a small cup, or a very controlled MIG with short bursts and a tight push angle. Back it with copper, set expectations with the owner, and consider replacing a larger panel if the metal has become brittle.
Second, contamination. Reefer interiors collect oils and residues you cannot see. Burning them into a weld creates porosity that looks like a sponge in cross-section. Clean longer than feels necessary. If a weld keeps bubbling, stop and re-prep rather than try to bury it.
Third, overhead or vertical-up. Roof patches often become overhead welds if you can access from inside. MIG overhead on aluminum is possible but messy without perfect settings and clean metal. TIG overhead is slow and requires patience. If time is tight, consider flipping the problem: remove a small section of interior liner to allow a more comfortable position, then replace the liner.
Heat is the quiet enemy. You can fix a crack and create three new waves of distortion if you pour too much energy into a thin wall. Staggered beads, air movement across the work, and cooling pauses keep panels flat. Clamping a heat sink behind the weld reduces draw. Plan the sequence: work from the center out, tack every few inches, and never weld the entire length of a long seam in one go.
Fire risk is real. Foam in a roof, dust in a wall cavity, or a forgotten rag behind a panel can light. Keep a watcher on the inside if you can. I carry two extinguishers and a bucket of water. Before I leave, I do a heat walk with an infrared thermometer, checking around the repair for hot spots, especially near wood framing and insulation.
Ventilation matters too. Argon displaces oxygen. Inside a closed trailer, a fan and an open door keep you alert. For stainless, watch fume exposure and use a respirator if needed.
If a side panel looks like a washboard from repeated welds and patches, welding may be a false economy. Longitudinal cracks that keep returning between rivets usually mean the panel is fatigued or the structure behind it has shifted. Replacing a full panel or two often restores stiffness and eliminates follow-on leaks. The calculus changes when graphics wrap the trailer, since re-skinning means rewrapping. Some owners choose a welded repair and new graphics on that section only, knowing it will not be perfect but will be functional.
Roof skins with widespread pitting from road salt or galvanic action at fasteners are also better replaced. Laying patches on Swiss cheese invites chases. A fresh skin, new sealant, and checked roof bows give another decade of service.
Owners who call for an aluminum patch often have other small jobs worth knocking out on the same visit. Railings on the interior sometimes need reinforcement. A gate latch or ramp hinge needs a new pin. Fence welding around the yard or a pipe repair on a wash bay line might be waiting. A mobile welder with stainless steel welding experience can handle a stainless threshold or an equipment bracket that cracked near the loading dock. Heavy equipment buckets and steps take MIG or stick, not aluminum, but the same truck can carry those tools.
For operations that live by uptime, bundling small fixes during a scheduled stop reduces the number of emergency calls. That said, emergencies happen. If a door gets ripped at 7 pm and a storm is lined up overnight, a quick MIG stitch and a cover plate can keep a fleet unit in service until a full day repair.
Most aluminum sidewall and roof repairs take one to four hours. A small crack with good access, TIG welded and sealed, falls at the low end. A roof patch that needs scaffolding and trim removal takes longer. Material costs are modest compared to labor, unless a large patch plate or new panel enters the picture.
Get photos to your welder before they roll. A picture of the damage wide and tight, plus a shot of the surrounding area inside and out, informs whether to bring extra staging, a second tech, or specific filler. Confirm whether the repair area is clean and accessible. If the trailer is loaded with pallets blocking the inside of the wall, mention it. The more a welder can plan, the shorter the downtime.
Anyone can lay a bead on a bench. Field work tests judgment. A welder who has worked across aluminum, stainless, and carbon steel in industrial and structural settings will read load paths, know when a neat TIG bead is enough and when a beefier MIG pass and a gusset plate are warranted. AWS certification does not guarantee artistry, but it signals familiarity with procedures and acceptance criteria. For structural connections at posts, headers, and hinge supports, that background matters.
After the weld, I do three things. I run a bright light inside the trailer against the repair and look from the outside. Any pinhole will show. I hose the area gently at first, then with more pressure, watching for drips. Finally, I drive a short loop if the owner is willing, to flex the structure and then recheck the seam. Aluminum moves. A seam that looks dry at rest might open slightly under twist.
Document the repair. A couple of photos and a brief note about filler, process, and sealant help the next mechanic, and they help you if a warranty question comes up. For fleet units, that note slots into your maintenance file alongside tire, brake, and pipe welding work records.
Trailers earn money when they roll. Sidewall and roof damage interrupts that. With a prepared site, a realistic plan, and a mobile welder equipped for aluminum, most problems become a half-day story instead of a season-long saga. The work is not glamorous, but a clean bead, a straight patch, and a dry interior are quietly satisfying. If you keep an eye on seals, avoid drilling through skins for ad-hoc mounts, and give the roof a quick scan after trips under trees, you will need fewer emergency welder calls and more deliberate, scheduled maintenance. That is the sweet spot for uptime and cost.
On Call Mobile Welding
917 J Pl Suite 2, Plano, TX 75074
(469) 750-3803