A paint film measurement tool pressed against old wood clapboard siding sometimes produces readings above 20 mils total thickness. That number is the accumulated weight of every paint job applied to the house since it was built, possibly spanning 50 to 70 years and half a dozen different paint technologies. At 20 mils and above, the paint buildup is thick enough to begin behaving as a rigid shell rather than a flexible coating. It cracks across grain lines, pulls away from the substrate as the siding swells and shrinks seasonally, and creates the alligatoring pattern, large scaly polygonal cracks in the paint surface, that signals the system has exceeded what can be sustainably recoated. Adding another coat on top of a 20-mil buildup does not solve the problem. It makes it heavier, more brittle, and closer to a full stripping event.

When Multiple Paint Layers Become a Structural Problem

The critical threshold in exterior paint buildup is 15 to 20 mils of total dry film thickness. Below this threshold, most paint systems remain flexible enough to move with the substrate through seasonal moisture cycling and temperature-driven wood expansion. Above it, the total film mass begins to exceed what the adhesion interface at the first primer coat can support, especially on wood siding where expansion across the grain runs 2 to 6 percent from dry to saturated conditions.

Alligatoring is the visible sign that total film thickness has reached or exceeded this threshold. The reptile-scale crack pattern forms because the outer paint layers are moving differently from the inner layers and the substrate. As the system ages and each layer loses independent flexibility, the cracks become deeper, cutting through multiple layers at once and creating pathways for water infiltration. Water entering through alligatoring cracks migrates to the siding-paint interface, freezes in winter, and accelerates delamination with each freeze-thaw cycle.

Not all thick paint systems present as alligatoring. Some thick systems hold together in a rigid shell until an acute stress event, such as a major temperature swing or a particularly wet spring, triggers widespread adhesion failure at once. When a paint system fails at multiple points simultaneously rather than progressively, the total stripping requirement appears more suddenly but is not more surprising given the total buildup.

Lead paint is a relevant consideration on any house built before 1978. Pre-1978 homes should be tested before any scraping, sanding, or mechanical removal of paint. The 3M LeadCheck Swabs provide results in 30 seconds and are EPA-recognized for wood, metal, plaster, and drywall surfaces. Finding lead in one layer of the paint system does not mean all layers contain lead, but any mechanical disturbance of a lead-containing layer requires compliance with wet scraping methods, HEPA containment, and EPA RRP rules for contractors working on pre-1978 properties.

Testing Adhesion on Multi-Layer Exterior Surfaces

Before committing to either stripping or repainting over existing layers, run a thorough adhesion test across multiple locations on the same surface. Paint thickness and adhesion vary across a single wall based on previous paint application method, wood grain direction at each board, and differential weathering patterns. A test result from one location does not accurately represent the condition at another location six feet away.

The cross-hatch adhesion test is the industry standard for this assessment. Score a grid with a utility knife using spacing of approximately 2 millimeters between cuts. The blade must cut through all paint layers to the substrate to produce a valid result. Press two inches of fresh painter’s tape firmly over the grid, rub it down with a fingertip, and pull sharply at a 90-degree angle. The tape must be fresh because tape that has been handled loses adhesion and underreports the amount of paint it lifts.

A clean result, where no paint or primer lifts on the tape, indicates that the existing paint system has viable adhesion and may be stable enough to recoat directly after preparation. A result where paint lifts in the grid pattern indicates adhesion failure at one or more layer interfaces. The depth of the failure matters. Paint lifting only in the outermost layer suggests that a single layer can be removed while inner layers remain stable. Paint pulling from multiple layers simultaneously indicates a system-wide adhesion problem that will not be resolved by adding another coat.

Run the cross-hatch test in at least five locations distributed across the wall: upper left, upper right, center, lower left, and lower right. Different areas of the same wall may have different adhesion status depending on how prior coats were applied and how that section weathers relative to the rest of the wall.

How to Decide Between Stripping to Bare Surface and Painting Over

The decision between stripping and recoating comes down to three factors: total film thickness, adhesion test results, and the condition of the topmost layer.

If the total film thickness is below 15 mils and the cross-hatch test shows no adhesion failure, recoating over the existing system after proper prep is a reasonable and cost-effective approach. Scrape all loose and failing paint, feather the edges with an orbital sander using 80 to 120 grit, clean the surface thoroughly, prime bare areas and feathered edges, and apply two topcoats. This approach works when the existing system is fundamentally sound and only the outermost layers have weathered.

If the total film thickness is above 15 to 20 mils, or the adhesion test lifts paint from any location, stripping is the correct approach. Adding another coat to a system with adhesion failures in the inner layers guarantees a future failure when the combined weight and stress of the additional coat exceeds what the weakest interface can hold. The Paint Shaver Pro is the most efficient mechanical removal tool for this situation on flat clapboard siding. It uses a carbide blade to remove paint like a planer, taking the whole system down to bare wood in one pass. All nails must be set flush before use because any protruding nail head will damage the carbide blades. After shaving, sand with 80-grit to smooth frayed wood fibers before priming.

Peel Away 1 from Dumond Chemical is the chemical stripping alternative for surfaces where the Paint Shaver Pro’s flat-blade geometry cannot access, including curved molding profiles, carved decorative elements, and intricate trim work. Peel Away 1 is effective for historic restoration work where the profile of original wood surfaces must be preserved under careful removal. Apply according to the manufacturer’s directions, maintain the recommended dwell time of 6 to 24 hours depending on the number of layers, and neutralize with Citrilize or white vinegar after removal. It works better in warm weather when the chemical action is more active.

Feathering and Priming Problem Areas on Heavily Painted Exteriors

On surfaces where some areas require stripping and others remain stable, the transition zone between stripped bare wood and intact painted surfaces requires careful feathering. An abrupt step from bare wood to the full thickness of the existing paint system telegraphs through the new topcoat as a visible ridge, especially under raking light or along horizontal clapboard lines where steps read clearly against the shadow of the lap.

Feather the transition with an orbital sander using 80-grit paper. Work outward from the bare wood area onto the intact paint, gradually thinning the existing paint layer over a span of 3 to 4 inches. The goal is a smooth ramp from zero thickness at the bare wood edge to full existing paint thickness at the edge of the feathered zone. After 80-grit feathering, follow with 120-grit to remove the scratches and create a surface that primer can flow over without pooling in the 80-grit marks.

Prime all bare wood areas with a high-quality exterior wood primer before the topcoat. Bare wood exposed by mechanical stripping is more porous than the primed surface at the feathered edge. Without primer on the bare wood, the topcoat absorbs unevenly across the transition zone, appearing blotchy and showing the boundary between stripped and intact areas even after two topcoats. Spot-prime all bare wood and all feathered transition zones, allow the primer to dry fully, and then apply the topcoat over the entire surface.

For clapboard siding, pay specific attention to the end grain at butt joints between boards. End grain absorbs primer and paint at two to three times the rate of face grain, and end grain that has been exposed through stripping or physical damage is the entry point for most moisture infiltration that leads to rot. Apply brush-coat primer to all exposed end grain before rolling or spraying the full wall, working the primer into the grain with bristle pressure rather than simply coating the surface.

How Incompatible Paint Types in Old Layers Cause New Coat Failure

Old exterior paint systems frequently contain layers applied from different eras with different chemistry. A house painted in the 1950s and 1960s may have lead-based oil paint in the first few layers, followed by alkyd paint applied in the 1970s and 1980s, followed by latex applied in the 1990s and 2000s. Each technology boundary in the stack represents a potential adhesion interface problem when a new coat is applied because the movement characteristics of each layer differ.

Oil-based and alkyd paints become increasingly rigid as they age and continue to oxidize. The outermost latex layers applied over rigid oil-alkyd substrates move with temperature and humidity changes while the layers below do not. This differential movement shears the interface between layers, producing adhesion failure that starts at layer boundaries and works outward. The cross-hatch test is especially important on old multi-technology systems because adhesion failure may occur at an inner layer interface rather than at the substrate.

Applying oil-based paint directly over a latex layer also causes problems in the opposite direction. Oil-based paint cannot expand with the more flexible latex beneath it, and the rigidity of the new oil coat cracks as the underlying latex substrate moves. The correct approach when painting over an unknown-type existing system is to use 100 percent acrylic latex, which provides the flexibility to accommodate movement in the layers beneath it regardless of what chemistry those layers contain.

If stripping reveals a layer of alkyd or oil paint that remains firmly adhered to the substrate after the outer layers are removed, that inner layer can be used as a substrate for new primer and topcoat if the cross-hatch test confirms its adhesion is intact. Sand the surface with 80 to 120 grit to create a mechanical key, prime with a high-quality acrylic bonding primer, and topcoat with 100 percent acrylic. Do not apply additional oil or alkyd over the remaining layer.