Fiberglass Pool Repair Underwater: A Complete Guide

Fiberglass pools provide a smooth, durable, and attractive addition to any backyard. However, even with proper maintenance, issues like cracks, leaks, and delamination can develop over time, especially in climates with extreme temperature fluctuations. Thankfully, many fiberglass pool problems can be repaired underwater without the costly process of fully draining the pool. With the right materials and techniques, underwater repair is possible for a range of damage.

This comprehensive guide will explore everything you need to know about repairing fiberglass pools underwater. We’ll cover how to assess damage, choose repair products, prepare the surface, apply epoxies and putties, and cure them properly. You’ll also learn about hiring a professional for more complex jobs, costs, temporary vs. permanent repairs, and when it’s time to replace the entire pool shell. Let’s dive in!

Hiring a Professional Fiberglass Pool Repair Company

When faced with a damaged fiberglass pool, many homeowners are tempted to tackle repairs themselves. However, there are some distinct advantages to bringing in a professional pool repair company instead.

Benefits of Hiring a Professional

Hiring professionals to handle fiberglass pool repairs underwater provides:

  • Proper preparation of the damaged area – Trained technicians have the skills to thoroughly clean, dry, and sand the surface for better adhesion.
  • High-quality repair materials – They use industrial-grade epoxies, putties, and resurfacers designed for long-lasting underwater repairs.
  • Correct repair techniques – Experienced crews know the precise application methods and curing times needed for optimal results.
  • Specialized tools and training for underwater work – Technicians have scuba gear, lighting, and other equipment to work efficiently underwater.
  • Warranties – Many professional fiberglass pool repair companies offer warranties on their underwater repair work, providing peace of mind.

Questions to Ask Potential Repair Companies

When evaluating professional pool repair companies, ask the following key questions:

  • How long have you been repairing fiberglass pools? Look for an experienced company with 5+ years focused on fiberglass pool repairs.
  • Are you certified in fiberglass pool repair? Reputable companies invest in certifications and ongoing training.
  • What methods and materials do you use? Ensure they use quality products designed specifically for underwater wet repairs.
  • Do you provide any kind of warranty? Opt for a company that guarantees their underwater repair work.
  • Can I see examples of past fiberglass pool repairs you’ve done? Ask for before and after photos to assess their quality.
Fiberglass Pool Repair Underwater

Costs of Professional Fiberglass Pool Repair

The costs of professional fiberglass pool repairs can vary substantially depending on the amount of damage and type of repair needed. Here are some typical price ranges:

  • Small cracks or holes: $200 – $800
  • Section replacements: $1000 – $4000
  • Full resurfacing: $6000 – $15,000
  • Additional costs: Leak detection, draining, cleaning, permitting – $500+

Minor repairs like patching cracks are at the lower end, while major overhauls like full resurfacing are at the top end. Get multiple quotes and ask what is included before choosing a repair company.

When a Full Replacement is Needed

While fiberglass pool shells can often be repaired, there are some instances where full replacement is needed:

  • Extensive damage – If cracks wider than 1/8th inch or damage spanning large areas of the pool surface.
  • Structural issues – Major stress cracks, delamination, or buckling point to underlying problems.
  • Deteriorated surface – If the entire gelcoat surface has oxidized and peeled away.
  • Leaks – Chronic leakage may mean it’s time to replace the shell.
  • Age – Pools over 10-15 years old may not be worth repairing.
  • Cost – In some cases, replacement is similar in cost to full repairs.

Consult a pool professional to assess whether repairs or replacement makes more economic sense for your specific situation.

Deciding Between Fiberglass vs. Concrete Pools

If you need to replace your existing fiberglass pool, one decision is whether to install a new fiberglass or concrete pool. Here are some pros and cons to consider for each material:

Fiberglass Pool Pros:

  • Smooth finish right away – no plaster curing needed
  • Low maintenance compared to concrete pools
  • Quick installation often in just days
  • Durable in cold weather climates with freezing temps

Concrete Pool Pros:

  • Highly customizable shape and sizes
  • Wide range of price points and options
  • Can achieve very large pool sizes
  • Natural stone look with plaster finishes

Key Differences:

  • Concrete generally lasts longer (20-30 years vs. 10-15 for fiberglass)
  • Fiberglass usually needs repairs and resurfacing over time
  • Fiberglass performs better in colder regions with freeze-thaw cycles

Analyze your climate, budget, and design preferences when deciding between fiberglass versus concrete for a new pool installation.

Assessing and Repairing Common Fiberglass Pool Damage

Now let’s drill down into specific techniques for identifying, preparing, and repairing the most common types of fiberglass pool damage using underwater methods.


Cracks are one of the most prevalent issues in fiberglass swimming pools. Thankfully, many cracks can be fixed underwater without draining the pool.

Identifying Cracks

Scan the pool walls and floor for any cracks. Look for:

  • Fine spider cracks, usually very short
  • Longer straight-line cracks from stress or impact
  • Jagged cracks from things like dropped tools or rocks
  • Discoloration around cracks indicating a leak

Preparing the Surface

Thorough surface prep is vital for crack repairs:

  • Clean and dry area vigorously with acetone
  • Scruff up surface with 220-400 grit sandpaper
  • Remove any loose debris or gelcoat
  • Widen crack slightly for better adhesion

Applying Underwater Epoxy

For small hairline cracks, an underwater epoxy repair kit is the best solution:

  • Prepare epoxy according to instructions
  • Thoroughly mix resin and hardener
  • Apply epoxy generously to ensure it fills crack
  • Use included putty knife to force epoxy into crack
  • Smooth excess epoxy over area about 2” around crack
  • Add additional coats until crack is filled flush
  • Allow to fully cure underwater before use

Results to Expect

Epoxy crack repairs should hold up well, but may need touch ups over time as shifting or cracks reappear. Larger stress cracks may require additional reinforcement.


Unwanted water leakage is another common fiberglass pool problem. Pinpointing the source of the leak and sealing it underwater is possible in many cases.

Finding the Leak

Identifying the exact position of an underwater leak can be challenging:

  • Look for dripping water on walls or deck
  • Use leak detection dye to isolate general area
  • Feel for moisture or eroded areas underwater
  • Listen for water rushing out with underwater headset
  • Professional leak detection may be needed

Applying Underwater Repair Putty

Once the leak source is located:

  • Clean and sand area vigorously with 220 grit
  • Force putty into the leak, applying pressure
  • Smooth and shape putty with putty knife
  • Apply additional layers until hardened plug forms
  • Allow full 24-48 hour cure time before filling pool

This creates a waterproof seal. Epoxy can also be used for leaks in some cases.

Preventing Leaks

Some tips to avoid leaks include:

  • Maintain proper pool chemistry to avoid erosion
  • Inspect regularly and fix cracks quickly
  • Update old or damaged underwater lights/fixtures
  • Reseal fittings, returns, skimmers if needed


Over time, air bubbles can form under the surface layer of fiberglass, creating blisters. These are largely superficial and can be repaired.

Identifying Blisters

Inspect the gelcoat surface for:

  • Small rounded bumps or bulges under the surface
  • Soft or spongy feeling when pressed
  • Smaller than a few inches in diameter
  • Often surrounded by a spider web pattern

Repairing Blisters

  • Drain fluid from blister with small puncture
  • Roughen surface with 220 grit sandpaper
  • Clean and dry area thoroughly
  • Fill void with underwater epoxy putty
  • Smooth putty flush and allow to fully cure
  • Apply gelcoat restorer to blend repaired area

This prevents blisters from worsening and restores a smooth finish.


Gaps or holes in the fiberglass pool surface need to be addressed to prevent leaks and further damage.

Causes of Holes

Holes or voids can form from:

  • Accidental impacts – falling objects, animals, debris
  • Improper installation – gaps in fiberglass sheet
  • Erosion from chemical imbalance

Patching Small Holes

For holes less than 2-3 inches wide:

  • Sand and scour hole edges with 100 grit sandpaper
  • Apply marine epoxy putty thoroughly into hole
  • Smooth putty level with surrounding surface
  • Let cure fully underwater according to instructions
  • Restore gelcoat finish over repair

Fixing Larger Holes

For larger holes over several inches:

  • May need to cut regular shape and install fiberglass sheet patch
  • Adhere with acrylic bonding putty or underwater epoxy
  • Apply gelcoat over patch to blend finish

This permanently seals damage from holes to prevent expanding issues.


Over time, the layers of fiberglass can separate, causing delamination damage.

Identifying Delamination

Look for:

  • Bubble-like bumps under the surface
  • Soft spots that flex when pressed
  • Whitish, crusty deteriorating gelcoat
  • Exposed rough fiberglass fibers

Repairing Delamination Damage

To repair delaminated areas:

  • Remove all loose, rotten gelcoat with putty knife
  • Sand damaged area with 100 grit sandpaper
  • Clean and dry surface thoroughly
  • Apply marine laminating epoxy resin
  • Lay fiberglass sheet over area
  • Apply more epoxy resin over fiberglass
  • Let cure fully before smoothing gelcoat over

This secures loose fiberglass layers and adds reinforcement.

Resurfacing Gelcoat

For widespread damage or deterioration, full gelcoat resurfacing may be needed:

When to Resurface

Consider resurfacing if:

  • Majority of gelcoat appears faded, chalky or has visible scratches
  • Multiple cracks, blisters and delaminated areas are present
  • Easier and cheaper than addressing individual issues

Resurfacing Process

Full gelcoat resurfacing involves:

  • Draining, cleaning and drying the entire pool
  • Removing all loose existing gelcoat
  • Lightly sanding the surface below
  • Applying colored gelcoat with rollers or sprayer
  • Spraying final clear coat layer
  • Wet sanding and buffing for a smooth finish

This restores the glossy, durable outer layer. Plan on $6000-$15,000 for professional resurfacing.

DIY Underwater Fiberglass Pool Repairs

Many underwater fiberglass pool repairs can be tackled as a DIY project if you have some repair experience and use the right materials. Let’s go over what you need to know.

Evaluating Your Skill Level

First, honestly assess if you are prepared to take on fiberglass pool repairs yourself:

  • Are you willing to put in the preparation work needed?
  • Do you have experience with fiberglass repairs?
  • Are you comfortable working underwater?
  • Can you follow instructions precisely?
  • Do you have the right safety gear?

If not, consult a pool pro instead.

Gathering Supplies

You’ll need:

  • Safety gear – gloves, goggles, mask/snorkel
  • Acetone or MEK cleaner
  • Sandpaper (80, 220 grit)
  • Scour pad or putty knife
  • Underwater epoxy or putty
  • Applicators like rollers or putty knives
  • Masking tape & plastic sheeting

Buy a specialized pool epoxy/putty, not standard epoxy.

Step-by-Step Application Process

Follow this general process for DIY underwater repairs:

  1. Identify and assess damage
  2. Drain any fluid from blisters/cracks
  3. Clean area vigorously with acetone
  4. Scuff area with 80 grit sandpaper
  5. Remove any loose material
  6. Widen cracks slightly with putty knife
  7. Apply masking tape around repair area
  8. Thoroughly mix epoxy/putty as directed
  9. Force material into cracks or holes
  10. Smooth and shape material over damage
  11. Add additional layers if needed
  12. Let cure fully submerged before removing tape
  13. Restore gelcoat or paint over repair if needed

Take care to follow all product instructions as you work. Rushing any steps can lead to failed repairs. Patience and precision is key.

When to Call a Professional

If you are unsure about taking on the repair yourself, call a professional. It’s also smart to hire a pro for:

  • Extensive damage spanning over 10 sq. ft.
  • Major leaks or structural issues
  • Replacing large sections of fiberglass
  • Full gelcoat resurfacing
  • Lack of time or confidence in your DIY skills

You want repairs done right the first time, so don’t be afraid to pay for a pro if needed.

Maintaining Your Fiberglass Pool

The best way to avoid the need for major fiberglass pool repairs is prevention through proper maintenance. Here are some key tips for keeping your pool in top shape:

Regular Cleaning

  • Use a high pressure cleaner to remove debris and buildup weekly
  • Vacuum the floor thoroughly at least twice a week
  • Scrub walls with baking soda or cleanser as needed
  • Shock treat routinely to prevent algae growth

Balanced Chemical Levels

  • Test and adjust chlorine, pH and alkalinity regularly
  • Shock treat after heavy bather loads
  • Closely monitor calcium hardness to avoid scale
  • Winterize properly to avoid freeze damage

Visual Inspections

  • Look over all pool surfaces monthly for cracks or blisters
  • Check for leaks, moisture or drips around the pool
  • Ensure lights, jets, filters are functioning optimally

Address any minor issues immediately to prevent bigger problems down the line. Consider having a professional inspect annually as well.

Surface Maintenance

  • Drain and clean entirely every 3-5 years
  • Wax or seal surface every 2 years to prevent staining
  • Repair small nicks, cracks and blisters promptly
  • Consider professional resurfacing every 8-12 years

Take care of your pool and you’ll minimize the need for those dreaded major repairs down the road. Enjoy the benefits of fiberglass for years to come with proper maintenance.


We’ve just covered everything you need to know about repairing fiberglass swimming pools underwater. With the right materials and techniques, you can fix a range of damage like cracks, leaks, blisters, and holes without draining the entire pool. In some cases, hiring a professional repair company is advisable for extensive issues or full resurfacing jobs. But for many problems, DIY repairs are possible if you have the skills and patience.

Just be sure to assess the damage thoroughly, prepare the surface properly, use a pool-grade epoxy or putty, and allow ample curing time. Address small problems quickly before they grow into major repairs. And most importantly, keep up with regular cleaning and maintenance to maximize your pool’s longevity.

Leave a Comment

Join Our Group!