refinishing terms

Refinishing Terms

Every industry has its unique language and set of terms. This section is devoted to providing definitions and interpretations for common words used in the bath and spa surface repair field. The goal is to establish a uniform, accepted meaning for each word as it relates to this repair industry. Hopefully, it will help to improve communications and understanding of the technology.


When used in reference to damage in a spa, bathtub, or shower, it means that the damage is limited to the surface. Most of these products are produced using some type of polymeric surface to provide the attractive aesthetics, which is reinforced with a stronger “backing” such as fiberglass reinforced polyester or strong polyurethane. Sometimes the surface has two layers. The second layer is normally another plastic such as acrylonitrile-butadiene-styrene (ABS).

A cosmetic repair is to re-establish the aesthetics of the surface so that the damage is undetectable, visually. Cosmetic damage can include blisters, cracks, crazing, burns, chips, and color change due to chemical attack.


Plastic (acrylic, gel coat, etc) spas and bathtubs are normally produced as a composite product. It will consist of a surface material to provide good aesthetics (sometimes as two layers) and a reinforcing material to provide the mechanical strength of the product. In addition, other materials, such as wooden supports and polyurethane foam, are often used to add strength to the product. When the integrity of these reinforcing materials is sacrificed, the structural strength is reduced.

This frequently requires procedures and materials to re-build the structure.


Blisters are round or oval bubbles on the surface or finish of a spa shell (rarely on a bathtub). Blisters were first experienced in the marine industry on the surface of gel coat/FRP boats. Technology was developed to solve the boat hull surface blisters, and it was adopted by spa manufacturers in the 1990’s. Today, blisters are a rare occurrence considering the thousands of spas built each year. They occur due to the formation of fluid pressure in a void space behind the surface in the composite reinforcement. They range in size from a dime size to the size of a plate or saucer. The average blister being about the size of an egg. There are multiple potential causes of the voids and the fluid pressure. Most blisters, when opened, are found to contain a colored liquid (mostly water).


Everyone recognizes what a material crack is. However, we distinguish between cracks and crazing, since they have a different appearance and somewhat different causes. A crack is defined as a single separation of a surface; whereas, a crazed surface has many micro-cracks spreading randomly like a spider web. There can be multiple cracks in a surface. Cracks are easily visible. Crazing may not be visible without magnification.

Causes for cracks are normally either an impact from an object or excessive stress from a constant load. They can also occur from fatigue caused by stress cycles (e.g. thermal expansion/contraction) Since the strength of a material declines as temperature increases, cracks occur more readily when the material is hot.


Crazing is generally associated with spas, but could occur on bath products. For example, crazing occurs on bathtubs, showers, and sinks made with either FRP (polyester resin reinforced with fiberglass) or cultured marble. Crazes form at highly stressed regions associated with scratches, flaws, stress concentrations and molecular inhomogeneities. The type of stress involved with surface crazing is most usually a tensile stress, and the micro-cracks generally propagate perpendicular to the applied tension. However, crazing can occur due to thermal stresses as well. Thermal stresses are induced due to expansion and contraction from temperature changes. This is the primary cause of crazing in gelcoat products. In most situations, total stress is comprised of both mechanical and thermal stresses. Crazing occurs mostly in amorphous, brittle polymers like polystyrene (PS), acrylic (PMMA), and polycarbonate (PC). The acrylic used for spas is specially designed to withstand the normal spa environment, but it will fail under harsh conditions.

Many studies by an acrylic manufacturer have demonstrated that two conditions must be present for stress crazing to occur on an acrylic spa surface. This may be due to the fact that acrylic used for spas is crosslinked, and therefore requires more energy to separate the molecules. The two conditions are high stresses within the acrylic and the presence of an aggressive stress-cracking chemical. The presence of only one of these conditions would not cause crazing.

Crazing appears as very small micro-cracks that look like a spider web on the acrylic surface. The stress tends to pull apart the tightly coiled polymer chains. This condition makes it easier for solvent molecules to penetrate the molecular structure of the acrylic and diffuse throughout the polymer chains. The solvent molecules act as a lubricant, which permits the polymer chains to separate from one another, creating very small cracks. Depending upon the amount of stress and the aggressiveness of the environment (i.e. chemical solvent), the small cracks continue to grow in size.

Styrene is one of the components of polyester resin reinforcement systems. It should be consumed in the polymerization process. However, in some instances there is residual styrene left in the composite structure. Since styrene is a strong stress cracking agent, it has been a common cause of crazing in an acrylic spa. However, chemical analysis must be used to definitely determine the cause of crazing in a specific sample.

Foam Injections

Polyurethane (PU) foam is commonly used between the back of a bath product and it’s base to provide mechanical support and sound and heat insulation. If the bath tub or shower base does not have PU foam, and exhibits poor strength, it can be injected into the cavity to improve its performance. PU foam is also used in spa construction for the same purposes. Low-density PU is used for insulation, and high-density is used for support.


Scratches are characterized by the fact that the “crack” does not go through the entire product wall. Generally, scratches are very shallow, and can be removed by simply sanding, buffing and polishing.


Plastic bath and spa products are of a composite structure consisting of a surface layer(s) and a reinforcement layer. De-lamination is the separation of these layers at their interface. This occurs since manufacturing conditions led to low or non-existent chemical adhesion of the two layers. In some cases a delamination or separation of the support or reinforcement wood, core or other materials can occur causing hollow or voided areas typically on the floor area of a composite bath tub or shower unit. This can occur in the footwell of spas, and on decks and/or panels or FRP boats and related composite products. This type of separation is not to be confused with fluid filled blisters. Delaminations will occur without the thick fluid associated with osmotic blisters. Water may be present associated to the usage environment.

Air Channel

Some bath and spa products are designed to allow “bubbling” air into the water from the bottom of the product. This is in addition to feature of hydrotherapy jets that discharge a mixture of water and air at high or low velocity or high or low volume. The ability to bubble air into the water is accomplished by installing a manifold on the back surface, and installing an air blower to blow air through it. Holes are drilled from the top surface into this manifold cavity.
The manifold is called an air channel.

Reverse Mat

This is the procedure/process of rebuilding the FRP support on the backside of a bath/spa surface when you can’t access it from the back. It entails pushing the FRP mat through a hole from the front and attaching it using polyester resin to the back surface.

Handicap Access

Bathing vessels are now being offered that provide easier entry by disabled persons. The most common bathtub designs incorporate a door in the side wall, which is opened to enter, and then closed prior to introducing water. Combination showers and bathtubs as well as shower-only units are offered in the market.

The American Disability Act (ADA) includes standards that products are required to meet in order to be ADA-approved.

Plastic Welding

Plastic welding, or hot melt welding, is the process of using a stick of polyethylene (normally), and melting it in a hot-melt gun to use as an adhesive for two surfaces.


Refinishing a spa or bath product describes the process and materials used to refurbish the entire surface of the product. Refinishing bathtubs and showers is a major industry in the United States. These practitioners of the trade develop their proprietary materials or buy standard products from a limited group of basic material manufacturers. They are typically epoxy-based or acrylic-polyurethane coatings. They each have their advantages and weaknesses.  Spa surfaces can also be refurbished, but have more limitations.

Buffing & Polishing

Buffing and polishing the surface of a spa or bath product is an identical process to what is used to increase the gloss of a painted automobile surface. Special compounds are available that perform a good job on typical spa and bath surfaces. Power buffing/polishing equipment used on autos is also used for spas and bathtubs. Materials and procedures recommended by repair material manufacturers and professionals should be followed.


In order to improve safety of bath and shower surfaces and minimize risk of falling on wet, slippery surfaces, manufacturers have developed various products and materials to apply to the floor surface to increase friction. Some products are simply appliqué materials that are stuck on the surface, and other products are available which need to be sprayed or brushed onto the floor.


Chemical products can attack and change the color of bath and spa surfaces. These are stains. Sometimes iron corrosion products are deposited on bath surfaces, which are referred to as rust.

Tub Bottom Replacement

When excessive damage exists in the bottom of a bathtub, but the rest of the tub is in good shape, a simpler method of refurbishment can be accomplished with a tub bottom inlay.

Drainage Problems

A common problem in bathtubs is failure to drain water completely. Two causes exist. The first is the bathtub was produced incorrectly and it did not establish an angle in the bottom to facilitate gravity drainage. The second cause is that it was installed incorrectly to facilitate gravity drainage. Both of these problems can be corrected with a tub bottom inlay.


Water leaks in spas and jetted tubs are predominantly caused by leaking pipes in the plumbing system. Even though there may be visible cracks in the surface, they do not normally result in a leak. The glass reinforced polyester resin that is applied on the back surface to impart mechanical strength will normally hold water. However, if water gets to this FRP, and there is un-reacted polyester resin, it can create blisters.

Leaks in a plain bathtub without jets are generally due to structural damage to a wall or bottom surface.