# Tapping a Sidewall: What Hollow Actually Sounds Like

> Tapping a laminated RV sidewall on a grid reveals bond failure as a dull hollow note against the sharp crack of bonded areas. The technique maps the true extent of delamination, which is typically two to three times larger than any visible bubble.

**Type:** Journal article  
**Canonical:** https://ocrv.pro/journal/finding-delamination-by-tapping-a-wall  
**Updated:** 2026-07-29

## Summary

Delamination is audible before it is visible. A knuckle tap across a grid finds separated areas as a dull hollow tone, often mapping damage triple the visible bubble.

A laminated RV wall is a sandwich. Gelcoated fiberglass on the outside, a layer of luan plywood behind it, block foam and aluminum or steel framing in the middle, and interior paneling on the back. All of it is bonded together under pressure with adhesive, which is what makes the wall stiff despite being light. When that bond fails between any two of those layers, the wall stops working as a unit.

Owners find delamination when they see a bubble or a wave in the sidewall, usually in raking afternoon light. By then the separated area is well established, and the visible portion is the part where the fiberglass has enough freedom to move. Around it is a much larger zone where the bond has failed but the panel has not yet moved enough to show.

You can find that larger zone with your knuckles. It takes an hour, it costs nothing, and it will tell you more about the actual condition of your walls than any visual inspection. Here is how to do it properly and what to do with the map when you have it.

## How to Tap and What to Listen For

Use the second knuckle of your middle finger, tapping firmly at a consistent force. Not a knock, not a slap. A crisp single strike. Bonded laminate returns a high, short, dead sound, like tapping a solid table. Separated laminate returns a lower, longer, hollow tone with a slight ring to it, because the fiberglass skin is now a free membrane over an air gap and it behaves like a drum head.

The difference is unmistakable once you have heard both within a few seconds of each other, which is why you tap a grid rather than probing suspicious spots. Move in a systematic pattern, roughly every four to six inches, working in horizontal rows from the top of the wall to the bottom. Consistency of force matters more than force level.

Some people prefer a plastic mallet or the handle of a screwdriver. Both work and both are slightly less sensitive than a knuckle, because your finger feels the response as well as hearing it. A separated area does not just sound different. It feels softer under the strike, with a slight give.

- **Consistent strike force:** Variation in how hard you tap produces variation in tone that has nothing to do with the wall. Steady, moderate, repeatable.
- **Four to six inch grid:** Close enough to catch a small area, coarse enough to cover a whole wall in an hour. Work in rows, not in random patterns.
- **Bonded tone:** Short, high, dead. No ring, no sustain. This is your reference sound and you should re-establish it frequently as you move.
- **Separated tone:** Lower, longer, with audible sustain and a softer feel under the knuckle. The skin is acting as a free membrane over a void.

## The Places Delamination Starts on Almost Every Coach

Bond failure needs a moisture path or a mechanical load, and there are predictable places to find both. Around windows is first, because window frames are large penetrations sealed with butyl tape that hardens and shrinks. Water gets behind the frame, wicks into the luan, and the adhesive bond fails from the inside outward.

Second is the lower third of the sidewall on the road side, because that is where stone and debris strikes happen, and an impact that does not break the gelcoat can still shock the bond loose behind it. On a coach that runs the I-5 corridor regularly, that band from about eighteen inches to four feet above the ground takes constant small impacts.

Third is any seam or joint: the corner radius where the sidewall meets the front cap, the slide opening perimeter, and the horizontal seam on coaches built with a two piece sidewall. Fourth is around every exterior fixture, because each one is a hole through the outer skin. Start your grid at these places, then expand outward until you find the boundary of the sound change.

## Turning Sound Into a Document

Tape works better than anything else. Blue painters tape, applied at the boundary where the sound changes, all the way around each separated area. When you finish the wall you have an outlined map you can photograph in one shot. This is the single most useful thing an owner can hand a shop or an adjuster, because it converts a subjective finding into a visual document.

Take a wide photograph of the whole taped wall with a tape measure in the frame for scale, then close shots of each outlined area. Note the date. If you repeat the exercise in six months, the growth or stability of each area tells you whether the problem is active, and an active delamination is a very different repair conversation than a stable one.

For claim purposes this matters enormously. Delamination following a documented impact is covered damage, and a tape map made shortly after the loss establishes extent at a specific date. Delamination discovered years later with no documentation is much harder to attribute to any event.

- **Boundary taping:** Painters tape at the sound transition line, outlining each separated zone so the entire wall reads in a single photograph.
- **Scale reference:** A tape measure in the wide shot lets anyone reading the photograph later quantify the area without being present.
- **Dated repeat survey:** Repeating the map at six month intervals distinguishes active spread from a stable historic condition, which changes the repair plan.
- **Moisture correlation:** A pin meter reading at the center of each mapped zone separates dry mechanical bond failure from wet, actively deteriorating separation.
- **Thermal confirmation:** An infrared survey in changing light confirms the tap map, since voids heat and cool at a different rate than bonded laminate.

## Dry Separation Versus Wet Separation

The map tells you where. A moisture meter tells you what kind, and the distinction drives cost more than area does. Dry delamination, typically from impact or thermal cycling, means the adhesive bond released but the substrate is intact. That is repairable by injection in many cases: adhesive introduced through small ports and the panel clamped or vacuum bagged back down.

Wet delamination means water got in and the luan substrate is degrading. Injecting adhesive into wet, softening plywood accomplishes nothing, because there is no sound material to bond to. Those areas need the skin opened, the substrate replaced, and the panel relaminated. That is a materially larger job.

Coastal coaches skew toward wet. The marine layer keeps everything damp for hours every morning through late spring, and a wall that would dry out overnight inland stays at elevated moisture content here. Owners in Shorecliffs, Capistrano Beach and the Riviera District see this pattern consistently, and it is why I always meter a delamination map rather than assuming.

## What the Repair Involves and What It Runs

Injection repair on a dry, well defined area is the least invasive option. Small ports are drilled through the outer skin at a spacing determined by the void size, adhesive is introduced, and the panel is pressed flat with vacuum or with a clamping caul while it cures. Then the ports are filled and the area is refinished. On a small area this is a manageable job.

Section replacement is the alternative when the substrate is compromised or the area is large. The outer skin is cut back to sound bond line, the luan is replaced, new adhesive is applied, and the panel is relaminated under vacuum. Then it gets faired, primed and refinished to blend with the surrounding panel.

Delamination repair runs $1,500 to $20,000 and up. Refinishing is a significant share of that on any coach with a complex paint scheme, since blending into full body paint is real work. Full paint refinish runs $5,000 to $35,000, which is why we always look hard at whether a repair can be confined within a panel boundary or a graphic break.

## What is included

- Delamination becomes audible long before it becomes visible
- A knuckle at consistent force is more sensitive than any handheld tool
- Work a four to six inch grid in rows, not a search of suspicious spots
- Window perimeters, the lower road side band and seams are the start points
- Painters tape at the sound boundary converts the survey into a document
- A repeat survey six months later distinguishes active from stable
- Moisture readings separate injectable dry voids from substrate replacement
- Coastal humidity pushes far more of these toward the wet category

## Questions

### How much bigger is the real delamination than the bubble I can see?

Commonly two to three times larger. The visible bubble is only where the skin has enough freedom to deflect. Around it is a zone where the bond has failed but the panel is still constrained by adjacent bonded material. A tap survey routinely turns a twelve inch visible wave into a thirty inch mapped area, which is exactly why the survey happens before any quote.

### Can delamination be injected instead of cut open?

Yes, when the substrate is dry and sound and the area is well defined. Adhesive is introduced through small ports and the panel is pressed flat under vacuum or clamping while it cures. If the luan behind the skin is wet and softening, injection has nothing to bond to and the area needs opening, substrate replacement and relamination. A moisture reading decides which applies.

### Does delamination get worse on its own?

Wet delamination does, reliably. The substrate continues to degrade and the void continues to spread outward from the moisture path. Dry impact delamination is often stable for years, expanding only under thermal cycling. Mapping the area with tape and photographing it, then repeating six months later, tells you which one you have without anybody guessing.

### Is delamination covered by insurance?

When it results from a documented impact or a covered water event, yes. When it develops from long term seepage or age, generally not. This is why a tape map and photographs dated shortly after a loss matter so much: they establish the extent at a known point in time. Delamination discovered years later with no documentation is difficult to attribute to any specific event.

## Related

- https://ocrv.pro/repairs/water-damage-and-delamination
- https://ocrv.pro/rv-fiberglass-repair
- https://ocrv.pro/rv-body-shop
- https://ocrv.pro/rv-paint-shop
- https://ocrv.pro/repairs/inspection-and-evaluation