Most joints on a car that is already built are made where the car is, not where the bench is. The section is too long to remove easily, the mounts are seized, or the owner needs the car back the same day. The joint gets made underneath, and its quality depends on preparation done before anyone lies down.
That preparation is the difference between a joint that lasts and one that is redone. Under the car there is no room to think, no space to fetch a tool, and no comfortable position from which to correct a mistake. Everything that can be decided in advance should be.
Cut the insert, bevel the ends, clean the faces and set out the tacks before the car goes up. Take the filler, the cup, the torch and the clamps to the car in one trip, and check the gas and the earth before you are lying on your back with the torch out of reach.
It also helps to rehearse the position. Sit or lie where you will be working and move the torch through the arc you will have to make. If the torch cannot physically travel around the joint, that is a fact worth discovering before the metal is hot.
Purging under a car is awkward but not optional on stainless. The usual approach is to tape a gas feed into the upstream end of the run, seal the downstream end, and let the gas displace the air before the arc starts. On a long system that takes patience rather than equipment.
Check the flow at the joint rather than at the regulator, because a long hose and a few joints will lose more than expected. If the inside of the seam comes out grey and sugared, the purge was not there, and the fix is not more heat but more gas.
Welding overhead or on your side changes the way the puddle behaves, because gravity is no longer helping. The pool wants to sag away from the joint, and the natural correction is to travel faster than the metal wants, which leaves a seam that is thin in the middle.
Shorten the arc, keep the torch angle steady and accept a slower travel than you would use at the bench. Where the position is at the limit of what a wrist can hold, stop, reposition and start again in a fresh spot rather than dragging the torch round and losing the line.
Above the joint there is usually something that must not be spattered: a fuel line, a brake pipe, a wiring loom, the underside of the floor. Spatter burns through insulation and heat travels upward, so a shield is part of the job rather than a nicety.
Damp cloth over what cannot be moved, a metal shield over what can, and a check afterwards that nothing has been cooked. A joint that is sound but has taken the insulation off a loom is still a repair that has to be done again.
The final check is done from the position the work was done in. Look at the seam all the way round, then look at the inside of the joint through the end of the run where that is possible. A seam that is bright outside and sugared inside will fail at the weld line rather than in the parent metal.
The techniques that make this reliable are the ones set out in TIG stainless steel welding, applied in a worse position than they were written for. Where a section has to be replaced rather than repaired, coming back with a new length of exhaust pipe is usually quicker than fighting a joint that has already been cut twice.
