# Tap the Panel Before You Panic, Then Learn What Hollow Actually Means > A knuckle and a roll of tape will map a void faster than guessing will. Here is how to tap a laminated RV wall in a grid and read the four sounds correctly. Source: https://ocrv.info/blog/tap-the-panel-and-learn-what-hollow-means Published 2026-02-11. Category: How-To Guides. About 5 minutes. Hollow means the outer skin has no core contact behind it. That could be delamination, a factory void from the original press, a wiring chase or a cabinet backing block. Tapping in a grid from known-good material outward and marking the boundary with tape tells you the size and shape, which is what determines the repair. ## The short answer Hollow is a report about contact, nothing more. It says the skin over that spot is not touching anything solid, and it does not say why. A grid tap works because it maps a boundary: you start where the wall is definitely sound, move in small steps, and mark the exact line where the tone changes. Shape and location interpret the sound. A round void mid panel and a long void following a stud bay mean two very different things. ## How to tap so the readings mean anything Consistency matters more than technique. If your force changes between taps, every reading is noise. 1. Pick your striker and stay with it. The butt of a plastic screwdriver handle is ideal. A knuckle works and is kinder to gelcoat, but your hand tires and your force drifts, so take breaks. Never use anything metal on a gelcoat skin. 2. Set your force by bouncing the handle off the panel from about two inches out, letting the weight do it rather than swinging. You want the same little drop every time. 3. Find known-good material first. On a sidewall that is usually low and forward, near the front wall corner where framing is dense. Tap it a dozen times until the reference tone is in your ear. 4. Work a grid, not a scribble. Roughly three inch spacing in horizontal passes, and keep the passes level. Ignore the temptation to jump straight to the spot that worries you. 5. Move outward from good material toward the suspect area, and mark with tape the moment the tone changes. One short piece of blue tape per transition point. 6. Circle the whole thing. Approach the suspect zone from all four sides, not just one. The tape marks will connect into an outline, and that outline is the finding. 7. Photograph the taped outline square to the wall, with something in frame for scale, before you pull the tape. Do this on a mild day. A hot skin over a void drums louder and will make the boundary read larger than it is. ## The four sounds, and what each one is telling you ### Bright and short A crisp click that stops immediately. Skin, adhesive and core are all in contact and behaving as one panel. This is what most of the wall should sound like, and it is your baseline. Do not skip establishing it. ### Dull thud with no ring Heavier, deader, but still tight. You are over a stud, a header, an aluminum tube or a run of framing. The panel is thicker and stiffer there, so it damps the strike. New owners hear this and assume rot. Tap a vertical line and you will find these repeating at regular spacing, which is the giveaway. Framing is on a layout. Damage is not. ### Hollow drum A tone with sustain to it, like knocking on a door. The skin is unsupported. This is the reading that matters, and it is the one you map with tape. ### Wet and dead Low, flat, no ring, and it feels different in your hand as well as your ear. The strike lands soft and the panel absorbs the energy instead of returning it. Saturated lauan does this. So does a block of foam that has crushed and packed down. A wet reading changes the whole job, because now you are looking at [water damage restoration](/services/interior-and-remodeling/water-damage-restoration) work inside the wall rather than a bond repair. ## Why a hollow spot near a window flange is not the same as one mid panel Location changes the consequence more than size does. Mid panel, out in open wall, a hollow area is a bonding problem. The skin is not carrying much there anyway. It looks bad, it will spread, and it gets addressed on its own schedule. Around a window, a slide opening, a storage door frame or an awning rail, a hollow reading means the substrate that fasteners and sealant depend on has gone soft or gone away. Those flanges are bedded in butyl and screwed into the wall. If the material behind them is a void, the screws lose clamp, the butyl stops being compressed, and the flange starts pumping water past itself every time the trailer flexes. A hollow band two inches wide following the bottom corner of a window is a much more urgent finding than a hollow patch the size of a dinner plate out in the middle of nowhere. That is when [reseal work on the opening](/services/body-glass-and-doors/window-repair-and-reseal) gets scheduled ahead of anything cosmetic. Corners deserve the same suspicion. Hollow along a vertical corner line usually means the corner post or the wall framing next to it has lost material. ## Three things that read hollow and are not damage Not every drum is a defect, and marking one as failure will cost you an unnecessary teardown. - Factory voids. Laminating presses do not achieve perfect adhesive coverage. Small, irregular, isolated hollow spots with a soft indistinct boundary and no distortion in the surface are often original. They tend not to grow, and they do not show in raking light. - Wiring and plumbing chases. Some builders route runs through a channel cut into the foam. That channel reads hollow in a straight line, often horizontal at outlet height or vertical dropping from the roof line. Straight, narrow, consistent width, and it lines up with a fixture on the inside. - Backing blocks and cabinet cleats. Blocking installed for a cabinet, a TV mount or a grab handle makes the surrounding area sound different because the backer is locally thicker. Open the interior cabinet and see what is screwed to that wall before you condemn the panel. The distinction that holds up: original features have soft boundaries and no growth. A failing bond has a defined perimeter, and it gets bigger between inspections. Mark it, date the photo, tap it again in a couple of months. ## How far you can take this yourself before a shop gets involved The tap test is genuinely good at one job, which is drawing an outline. It cannot tell you moisture content, it cannot tell you whether the core crushed, and it cannot see whether a frame member behind the skin has corroded. Owners in Eastvale and out toward Ontario who bring a taped and photographed outline with them are handing over real diagnostic work, and there is no reason to pay a shop to repeat it. Where you stop is at penetration. A pin type moisture meter reads through the interior wall board without touching the exterior skin, and that is a reasonable next step you can do yourself in a closet or behind a dinette panel. Drilling the outer skin is not. A hole in gelcoat is a permanent repair even if the news behind it turns out to be good. Past that, the honest next move is a small exploratory access made from the inside, which means removing an interior panel and looking. That belongs to whoever will be doing the [structural evaluation](/services/collision-and-structural/structural-damage-assessment) anyway, because they will choose the access point based on what they intend to rebuild. --- OCRV Center. 23281 La Palma Ave, Yorba Linda, CA 92887. (949) 799-3387. info@ocrvcenter.com. Serving Eastvale, California in Riverside County, about 20 miles from the shop. All work is performed in shop at the Yorba Linda facility. No mobile, roadside or on-site service is offered. Scope is body, paint, structural, fiberglass, interior and vehicle systems work. Engine rebuilds, transmission rebuilds, drivetrain work, DOT inspections and emission testing are not performed.