This guide will assist in repairing a variety of Centrex surface issues. All spa surfaces are subject to damage, and techniques for repairing scratches, gouges and blisters will vary from material to material. The surface makes all the difference when the need for repair arises. Please read this guide carefully before proceeding with your repair.
Centrex expands and contracts faster than acrylic. When the spa is exposed to sunlight or a high temperature difference, such as when filling your spa on a cold day with hot water, it can cause surface cracks. Even if the spa is used normally, after a number of years it can develop surface cracks.
Even the most unsightly cracks do not usually harm the structural integrity of the spa, as long as they are only skin/Centrex deep. For deeper cracks or punctures that penetrate the fiberglass backing, the procedure is slightly more involved. Once the integrity of the fiberglass backing has been restored, repairing the surface is the same as any minor crack.
Blisters are most often caused by osmosis. The Centrex has delaminated but not because it didn’t bond to the fiberglass, rather by the process of osmosis (the tendency of water to travel from a less salty to a more salty environment to evenly disperse the salt content).
Water molecules penetrate the surface of the spa, passing through the larger Centrex molecules and moving into the laminate interface and the fiberglass.
Once it reaches the fiberglass, the water settles into air pockets and begins to break down the fiberglass, creating a thick fluid. Once the water has bonded to the resin and fiberglass, the molecules are too large to pass back through the Centrex shell and the fluid build up raises the Centrex to become a blister.
Use in a well ventilated area. Wear safety glasses or goggles and protective gloves. Follow the safety instructions as directed on the warning label of the MEK packaging.
Materials & Equipment Needed
The spa must be clean and the repair area covered from rain, sun and dirt.
Prepare the repair area by wiping it clean with paint thinner and masking off the damaged area with masking tape. This will protect the surrounding Centrex from beads of MEK that may run off the repair and cause further damage.
This will suffice for minor scratches.
For deeper scratches, cracks and blisters, prepare the filler ahead of time by mixing Centrex pellets and MEK in an open glass container (sealable) until they are dissolved and mixed. The cap should be sealed on the container immediately and remain closed until the mixture is ready to be used or else it will dry prematurely.
Begin preparing the mixture one hour before use to allow time for the Centrex to dissolve.
Removing a scratch is a matter of sanding the damage smooth and then feathering the area out into the undamaged spa surface with wet /dry sandpaper. Next, read the paragraph at the end of this guide entitled “Blending the surface texture” for the final steps.
In repairing cracks, it’s necessary to drill a small hole at each end of the crack to prevent it from growing longer. The hole should only be as deep as the crack and not go beyond the Centrex. Be sure not to drill through the fiberglass shell.
Chips and/or cracks can be prepped using a Dremel tool equipped with a small cutting bit. Cut away enough of the Centrex to accept the repair material. After the area has been gouged with a Dremel, clean it out with an Exacto knife and wipe the area clean.
Either use the Centrex mix which you have created or drop a small amount of MEK on the Centrex tile material. (The MEK will melt the surface on contact) and mix it with the Centrex until you have a consistent paste.
With a metal spatula, gently scrape the Centrex tile surface or use the previously prepared Centrex MEK paste until a sufficient amount of Centrex is on the end of the blade.
Wipe the Centrex material onto the spa’s damaged surface, Repeat the process until filled.
Once filled and dry, scrape the excess off with an Exacto blade.
Let stand to cure (up to 1 hour at room temperature) Sand to smooth once the Centrex has fully dried. Sanding is complete when you can run your hand over the repair and not feel it.
Drill a 1/16th of an inch hole at the top of the blister. A brown fluid will begin to leak out.
Heat the affected area with a heat gun or hair dryer until the Centrex becomes soft. Heat an inch outside of the repair directing the heat towards the center of the repair.
Use a vacuum pump or Shop Vac at the hole to vacuum seal the repair and reform the Centrex. More brown fluid will continue to escape the blister and the Centrex will return to its original shape. Centrex has a memory and has a natural tendency to return to its original shape.
Cover the 1/16th of an inch hole with the Centrex mix. Remove the excess when the mix has dried with an Exacto blade and blend in the repair. Do not sand.
Read the paragraph at the end of this guide entitled “Blending the surface texture” for the final steps.
Prepare area to be filled by wiping it clean and masking off the damaged area with masking tape.
This will protect the surrounding Centrex from beads of MEK that may run off the repair and cause further damage.
Once the blister has been opened up, and the fluid is removed, apply heat directly to the affected area. With a heat gun, heat the fiberglass until the wetness in the fiberglass starts to pop and dry out. If it is not dry, the fluid will come back through the repair and cause it to fail.
Once the fluid has been removed and the affected area is dry, sand the area with a drill and a small rotary sander as shown in the picture. Sand the area past the Centrex until the affected areas are removed and all that is left is cured fiberglass.
Next, prepare the fiberglass mixture and lay into the affected area until it is approximately 1/8th of an inch from the surface. Sand smooth once the fiberglass has cured. Then layer with Centrex.
The Centrex needs to be applied, allowed to dry, sanded down and reapplied a few times (typically three) to ensure a solid repair. The drying procedure can be accelerated with a heat gun or hair dryer, but if the Centrex dries too fast it may shrink. Aim the heat around the repair, not directly on it, to prevent pinholes from developing in the Centrex.
Allow each 1/8th inch layer of Centrex to cure before sanding and applying the next layer. The last layer should go above the surface level to allow for shrinkage.
Next, read the paragraph at the end of this guide entitled “Blending the surface texture” for the final steps.
There is no guarantee that a blister repair will last or that more blisters will not appear.
Vertical, Inclined or Curved surfaces
To repair surfaces which cannot be positioned in a horizontal or level plane, it is necessary to create a pouch or pocket to contain the viscous material until it is cured. After preparing the damaged area, use clear packing tape to fabricate a small pouch or pocket over the cavity to be filled. Slowly pour or use the mixing stick to drip the repair mix into the pocket created with the tape. After filling, leave the top of the pocket open to allow any air to escape. Allow the repair material to cure, remove tape, sand and polish.
Blending the Surface Texture
To blend the sanded surface back into the rest of the spa almost seamlessly, use only Methyl Ethyl Ketone (MEK). MEK is the only material that you can mix with Centrex and get it to break down without affecting its color, as long as it is used in a well ventilated area.
The most effective method for bringing the texture back to a Centrex spa surface is to put a small amount of MEK in a metal or ceramic cup (not plastic), dipping in a paint brush and carefully use the brush to dampen the damaged surface of the Centrex. Then simply press on the bristles, leaving a dimpled impression on the softened spa shell. If the texture is too deep, simply sand some off and re-texture if necessary. Dip the paint brush so that it’s just moist, not dripping. This method is essentially stamping the material as soon as the MEK has softened it.
A textured sponge can also be used to create the imprint. After the MEK is applied, gently press on the affected area until the desired texture is achieved. Start from the outside of the repair and slowly work your way in.
Let stand to cure (up to 4 hours) DO NOT SAND.
At the end of the repair, there may be shiny spots peeking out through the dappled texture. These can be eliminated by spraying on a very light coat of MEK. It will blend the shine in the repair onto the original. The MEK acts as a finishing solvent when used this way.
Re-filling the Spa
If the spa is outdoors in a cool climate, the filler and coating could take up to seven to ten days to cure. Ideal ambient temperature is between 65 and 72 oF. Refilling the spa too soon could make the Centrex peel. Ensure hot water is not used to fill an empty Centrex tub as this will cause surface cracks.
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