Decide per panel, not per vehicle
The exterior of a passenger car is not one surface. A painted steel roof, a glass sunroof, an unpainted bumper, a rubber door seal and a chrome badge each carry a different deposit, at a different thickness, and they lose heat at different rates. Choosing a single mode for the whole car means the operator either over-wets the areas that must stay dry or under-treats the areas where the deposit is bonded. Write the panel list first, choose the mode against the deposit on each line, and let the vehicle be cleaned in mode changes rather than in one pass.
Before any lance moves, the two modes need a plain definition that the whole team uses. Dry steam is heat delivered with very little free water, so the panel gets energy rather than a soaking. Wet steam and the hot-water mode carry free water with them, which gives dwell time and physically lifts a deposit that has dried onto the surface. The site pages for the cleaning range record that many units switch between dry and wet steam, and that the mobile units are additionally listed with a hot-water mode; check the page of the exact unit being quoted, because the listed modes are not identical across models, and never assume a mode the quoted page does not name.
What dry steam and wet steam actually do to a panel
On a clear coat, heat does most of the useful work. Traffic film, waxy dressing residue and old polish overspray are deposits that soften when they are warmed, and they can be lifted without the panel being flooded. That is the argument for dry steam on painted flats, on horizontal glass, and anywhere free water would sit in a seam or run into a shut line. Little free water also means a shorter drying step, which is the difference between a car that can be handed over in the same shift and one that has to wait for a panel to dry out.
Free water earns its place when the deposit is thick and bonded. Road grime packed into a rocker panel, brake dust baked onto the inner face of a wheel arch, tree sap on a bonnet edge and salt crust left by winter road treatment are not warmed loose in one pass; they need dwell, and dwell needs water. Wet steam and hot water also matter where the operator must see what has been removed, because a filmed surface is difficult to inspect while it is still carrying deposit. The trade-off is run-off and standing water: every wet pass creates water that has to drain, dry, or be managed under the site's drainage rules, and that is a real cost on a forecourt or an indoor bay.
Roof, glass and horizontal surfaces
Roofs, rails, sunroof glass and the horizontal face of a bonnet collect airborne film rather than sprayed-up grime: fallout, tree sap, and the dull haze left after repeated washing. Heat handles this well, so start dry. Two conditions change the answer. A roof that has been waxed or dressed recently carries a softened film that a dry pass will move rather than remove, so a follow-up wet pass and a wipe may be needed. And a glass panel with a moulded edge trim or a bonded sunroof needs the trim read as an assembly, not as glass: heat near a bonded joint is a question for the vehicle's own documentation, not something an operator decides in the bay.
The reason to be deliberate here is that horizontal panels are where free water waits. Water driven onto a roof sits until it drains somewhere, and it will find a sunroof channel, an antenna base or a rail joint on its own.
Painted flats: bonnet, doors, wings, quarter panels
These are the areas a customer judges, so they are also the areas where the mode choice is most visible. Work dry steam over painted flats, keep the lance moving, and let the heat soften the film while the operator lifts it with a wipe. Two panel conditions stop the mode discussion and are written here as shop practice that the buyer should have in writing before steam reaches the car: a panel that has been corrected, polished, or re-coated recently, and any area where the coating is open, lifting, or uncured. Heat on an uncured or freshly finished panel is a decision for the person who did that work, and a detailing operation that cannot get that answer should exclude those panels from steam work entirely and clean them another way.
Where a flat does carry bonded road film, a short wet pass ahead of the dry pass is a normal sequence: water to lift, heat and a wipe to finish. Record the order in the method so it can be repeated rather than rediscovered.
Lower panels, rockers, arches and wheel surrounds
This is the wet-steam part of a car. Rocker panels, the lower edge of doors, the inside face of wheel arches, splash flaps and the rear of the front bumper carry sprayed-up grime, brake dust and, in winter, salt. The deposit is thick, and it is shaded from drying, so dwell time matters more than the risk of water. Wet steam or hot water first, and an operator who expects to finish this part of the vehicle damp.
Two boundary conditions belong on the sheet for this band. Brake components, calipers and the friction face of a disc are not a cleaning target for the operator unless the vehicle owner or workshop has asked for them, and water driven into a hub, a bearing seal or an electrical connector underneath the car is the failure to plan for, not the deposit. Route the run-off from this band deliberately: on an outdoor forecourt it leaves the pad, and indoors it reaches the site drainage.
Bumpers, unpainted trim, and unprimed plastic
Textured unpainted plastic behaves differently from painted steel. It is softer, it is often a matte or recycled surface, and on an older vehicle it is frequently already chalked or crazed. Heat is still the useful part of the process here, but the operator should treat the first contact as a test rather than as a pass: work on an inconspicuous area, keep the lance moving, and watch the surface for change. A bumper cover that has been repainted in a body shop, or one with an adhesive badge on it, needs the same caution as a corrected panel.
For this band a written answer is worth more than a rule of thumb. Ask the supplier, in the enquiry for the customized steam cleaning equipment, what the machine is expected to be used on across the vehicle mix the site actually sees, and ask for the guidance to be written into the method rather than described verbally.
Badges, emblems, decals and aged vinyl
Small applied parts are the most common source of an expensive mistake. Badges, chrome emblems, pinstripes, body decals and vinyl wrapping are held on by adhesive, and adhesive responds to heat. On an older decal, or on wrapping with lifted edges, the operator is not cleaning a surface but loading the bond that holds it down. The safe instruction for a shop is a short one: exclude applied parts and any wrapped area from steam work, clean them by hand, and put that exclusion into the work instruction so it does not depend on the operator noticing.
Where a badge or a decal has to be worked around, plan the panel as one area with a hole in it. The dry pass goes up to it; nothing goes at it.
Seams, shut lines, jambs and drain paths
Every exterior panel has an edge, and at that edge there is a gap the operator cannot see into. Door jambs, the shut line of a bonnet and boot, the underside of a spoiler, roof rails, and the channels under trim are where free water is collected rather than shed. For those areas the answer is almost always dry: heat reaches the deposit without adding water that has nowhere to go. A wet pass along a shut line pushes water into a jamb that a wipe cannot reach later, and a panel that looks clean at handover can still be holding water behind a trim clip.
The practical version of this rule is a column on the sheet that reads water in, water out: for every area where a wet mode is used, name where the water goes and how it is verified dry. Where the operator cannot answer that, the mode reverts to dry.
The two switch tests that settle the choice
Two tests are worth running on the site's own vehicles before the choice is fixed for everyone. First, a deposit test: pick a vehicle with a mixed exterior, treat one comparable area dry and another wet, and let the team record which area cleared and what it cost in drying time. Second, a moisture test: after each mode, check the areas that stay wet, the shut lines and any area under trim, and write down whether they were dry within the shift.
Because published machine pages describe modes rather than results on your vehicle, these tests decide the question. A specification sheet says which modes a unit offers; it does not say what a salt-loaded rocker or a wrapped roof does in the bay. A bounded trial on the vehicles the site actually cleans produces the rule that the method document then records.
Exclusions that belong on the operator sheet
Put the exclusions in the same document as the mode choice, so an operator reads them in the same breath. The list for a passenger car exterior is short: wrapped, re-coated, corrected or open paint; decals, vinyl and applied emblems; brake friction surfaces and anything the owner has not asked to be cleaned; areas where the vehicle's own documentation limits heat or water; and on a vehicle of any age, trim and seals that are already lifting.
State the exclusions as named areas rather than as a number, and give the fallback for each: what the operator does instead when steam is not the method. A rule without a fallback gets ignored the first time it blocks a job.
Recording the choice with the settings
A mode choice that is not recorded is not a standard. For each panel group, note the mode, whether a wet pass preceded the dry pass, the lance distance and dwell as the operator used them, and the drying result at the end of the job. Alongside that, ask the supplier for the rows on the quoted unit in writing: the modes that unit offers, the water inlet arrangement, and any limits the page states about where those modes are intended to be used.
A number printed on a product page is a characteristic of that machine, not a result on a customer's car, and it says nothing about the vehicle mix, the drainage or the shift pattern of the site doing the cleaning. Treat the page as the start of the enquiry and the site trial as the answer.
Hand-back check before the vehicle leaves
Before the car leaves the bay, walk it with the panel sheet in hand: are the areas that were worked wet actually dry at the shut lines, jambs, arches and trim edges, and are the excluded areas still excluded? Confirm that the deposit has gone and the moisture has not stayed.
The hand-back check closes the choice loop. If a wet pass was used on an area that cannot be verified dry, the sheet is corrected before the next vehicle arrives, and that correction, not the original decision, is what makes the mode choice repeatable across a team.
