Tag Archive: crack

TO: Multi-Tech Products:

I am a repair technician; I have inspected a cultured marble whirlpool with a 4″ vertical crack on the tub. The crack is on the bottom radius around 12″ from the drain. The tub has leaked through due to the crack.  Do you have a method/product that we can use to repair cultured marble and guarantee that the tub would no longer leak.

Any help you can give us would be greatly appreciated.  We look forward to hearing back from you.  If you have any questions, please don’t hesitate to contact us. Otherwise, have a wonderful day.

All the best,

TO: Maureen

You describe a thermal stress crack created by the shock of hot and cold water.  I have seen this type of crack go completely around the perimeter of the tub.

In order to repair it, and prevent future propagation of the same crack, you need to drill a hole at each end of the visible crack.  Be sure to drill through the entire depth of the tub with a ¼ drill.   Then grind a deep slot into the structure along the entire length of the crack. Just grinding an 1/8th inch groove in the top surface is insufficient since it will continue to crack. This is because the crack goes through the complete structure. The surface will continue to expand and contract as it is exposed to hot and cold water. This expansion and contraction will lead to a new crack directly through the repair. Cultured marble bathtubs aren’t reinforced with glass fibers, so they have less strength to resist failure from expansion cracks.   If there is access to the back of the bathtub, you should attempt to add reinforcement using fiberglass mat.  The combination of reinforcement on the back side and grinding the crack open through the structure will provide the best permanent repair.

The fact that this is a relatively short crack improves the probability that this crack can be satisfactorily fixed.

If there is no way to get glass on the backside, aggressive grinding and heavy glass reinforcement from the top side will achieve a good repair. The repair procedure should be:

  1. Grind the crack to create a recess in the top surface along the crack wide enough ( probably 2 to 3 inches) to accommodate the fiberglass mat.
  2. Using a heavy duty die grinder, grind a slot channel that would hold a recessed layer of heavy duty stitch mat. The stitch mat would be about an 1/8th of inch thick. A slot grove would need to be ground ¼ inch deep to hold the mat and the resin while leaving enough room for the poly filler after the mat and resin application. The slot groove would not go through the structure, but leave enough structure to bond to. The slot groove would hold the width of the Stitch mat to straddle the crack a little over an inch on each side of the crack. If the crack is 1/16th inch by 12 inches. The slot groove should straddle the crack by 1 ¼ on each side and extend past the end of the crack the same. Total groove over the crack 2 ½ inches x 14 ½ inch long by ¼ deep.
  3. With the groove and the crack prepared to except the fiberglass application, apply catalyzed Iso resin to the glass first and place it in the groove.  Smooth out and remove bubbles.
  4. Force cure with a heat gun.
  5. Fill with Poly-filler.
  6. Use our Quick Glaze system with the Clear Coat to achieve better color match and appearance.

You should charge your customer between $350 to $450 for this best-practice job, and it should take 3-4 hours to complete.

Hope this helps.

Rob Clos


TO:  Multi-Tech Products

I have a spa that has developed crazing in the acrylic surface.  See the photo.  How can it be repaired?



TO:  John

Repairing a large, crazed area is complicated, since it requires removal of the affected acrylic surface.  A hammer and chisel is normally required to remove this material.  Be carefult not to punch a hole completely through the wall structure.  Then our Acrylic Filler and color-matched coatings are required to refinish the area.  Refer to the procedures for repairing spas with the Quick Glaze system.  Often, the cause of crazed acrylic is the presence of trapped styrene in the polyester resin backing system.  So to prevent re-occurrence, you should use a heat gun to evaporate any residual chemicals in the exposed FRP prior to starting to apply the acrylic filler.  See our explanation of crazing in the website, http://www.multitechproducts.com.

Remember that crazing affects only the appearance of the surface.  It will not result in water leaking through the structure.  In some instances, where there is minimal crazing, it can be softened with heat.  Using a heat gun, you would heat the surface up to about 220ºF, which will cause the crazing to relax and reduce the severity of the micro-cracks.  One acrylic manufacturer suggests that wiping the area with a paste wax, or equivalent, sometimes works to fill in the micro-cracks to make them less visible.  You could try some of the MTP paste repair kits.  Using a cover on spas is very important in avoiding these and other surface issues.

I hope this has been helpful.

Rob Clos

Fixing a crack at a spa jet

To:  Multi-Tech Products

My spa has a crack at one of the jets, and water is leaking through. Will your acrylic granite repair  kit permanently seal the crack?  How can I remove the jet to make the repair?  It seems to be anchored very tightly.



Jet Crack






To: Dennis

Obviously, you have a crack in the acrylic surface, and you report your spa has evidence of water leaking into the rear area of the shell.  Although it might be possible for the two events to be related, generally they are not.    The reason that most cracks in the acrylic surface do not lead to a water leak is that spa shells are constructed using at least two layers of different materials.  The acrylic surface is only used to provide the attractive color and effects.  This outer shell is produced from an acrylic sheet that is heated and formed (stretched) by vacuum into a mold.  The original sheet is only about .125″ thick, so there are areas in the bottom where the acrylic thickness is between 0.020″ and .030″ thick.  So most spas are sprayed with a polyester resin filled with chopped fiberglass strands (FRP) to provide the strength required for the shell.   Also, most manufacturers then spray a coating of a high density polyurethane foam on top of the FRP for insulation.  The FRP reinforcement is very strong, and it is adhered to the back of the acrylic.  It would be very unusual for this FRP structure to crack, which would be necessary to lead to a water leak.  Although the acrylic surface of spas are known to crack for various reasons, water is still contained by the FRP structure.  If you can actually see water leaking behind the crack, it would be one of those one in a million situations.  However, since there is a hole in the shell at the jet, it might be possible for the water to find a path through that hole.  Typically, silicone sealants are used to prevent leaks at the jets.

The most common source of water leaks is the plumbing.  These leaks are often difficult to find due to the insulation in the spa cavity, especially when the cavity is completely filled with polyurethane foam.  You may need to get a spa maintenance expert to locate and repair a leak in the plumbing.

However, to answer your question about the acrylic repair, our granite repair kits are designed for this purpose.  Please refer to our website, http://www.multitechproducts.com/pages/Procedures.html, for a description of the materials and procedure for the repair.  The granite repair kit will repair the crack up to the jet, and without removing the jet.   If the jet needs to be removed, you need to consult with a spa mechanical repair contractor or the manufacturer.

I hope this will answer your questions.

Rob Clos