How To: Fiberglass Blister Repair


Is a boat blister ruining your shiny gelcoat? Don’t panic!

When water seeps underneath your boat’s gelcoat, the pressure creates dome-shaped protrusions on the hull. That’s how osmotic blisters, sometimes called boat chickenpox, are formed. 

A blister will not necessarily threaten the structural integrity of a boat. In fact, more than 95% of boats with fiberglass blisters do not develop any other issues. Yet it cannot be denied that a blister-ridden gelcoat is quite unsightly, and can detract from the resale value of a boat.

In many cases though, the blister repair is inexpensive and does not even require a boat repair expert. If left untreated however, blisters can be dangerous. 

Why Do Boat Blisters Happen?

Boat blisters arise due to the chemical interplay between the gelcoat, the underlying laminate layers and a process known as osmosis. Let’s break that down:

  • The gelcoat of a boat is the glossy exterior surface that shields the hull. It offers some degree of structural protection, but that is not its major purpose as it is often permeable to water. With age and abrasion, the gelcoat may erode and wear off. However, it can be restored
  • Underneath the gelcoat are the layers of laminate, which make up the core structural components of a vessel. They are built from resin-saturated fiberglass, which is quite waterproof, although small quantities of water may pass through.
  • Osmosis is the passive diffusion of water through the semipermeable membrane of the gelcoat and fiberglass. When the water goes in, it gets trapped by loose materials within the laminate. These hydrolysable materials such as excess pigment, dust or dirt from the boat’s fabrication may form concentrated acid solutions that draw in even more water.

Eventually, the area under the gelcoat becomes swollen and blisters form on the exterior surface of the hull. Blisters may also appear inside the boat if water is left to collect in the bilge. 

Although blisters occur in one of every four fiberglass boats, they are rarely anything more than a cosmetic issue. Moreover, minor blisters are easily treated.

How to Repair Boat Blisters

Generally, boat blister repair requires the following materials:

  • A chisel or screwdriver
  • Protective goggles
  • Epoxy resin
  • Colloidal silica, as filler
  • A quart of acetone
  • Trisodium phosphate (TSP)
  • Acid brushes
  • 36-grit sanding disk
  • A yard of 6 – 10 ounce fiberglass cloth (for deep blisters)

Do not attempt to use polyester resin instead of epoxy. Polyester is not adhesive enough and does not provide adequate waterproofing. Similarly, microballoon fillers should not be used in place of colloidal silica if you want a lasting treatment.

The procedure for repairing minor hull blisters is to first pop them to let the liquid drain out before grinding the area. The depression created is then washed, dried and filled with resin before fairing and painting over the area. 

For deep blisters that have penetrated the laminate, you will have to replace the damaged fiberglass before applying epoxy resin. Essentially, blister repair can be performed in five steps.

Pop ‘em open & let ‘em drain

Blisters are most apparent when the boat is initially hauled out of the water, but they often shrink afterwards, sometimes in less than an hour. Circle them with a marker just in case this happens. 

The liquid contained in a blister is acidic and gushes out under pressure. Use a chisel or screwdriver to pop the blisters and let the foul liquid drain completely. It’s a good idea to hold the chisel at arm’s length and wear protective goggles, for your own safety.

Grind the area

Using a disk grinder loaded with your 36-grit sanding disk, grind the open blister into a depression. It should be no deeper than the last damaged layer of laminate beneath the gelcoat. A rule of thumb is to grind the depression 20 times wider than it is deep. 

You should not limit grinding to the open blister, but also to the surroundings. With the handle of a screwdriver or chisel, tap around the blister pocket. Damaged laminate will give a noticeably dull report, while firm laminate will sound sharper. The depression should enclose the entire delaminated region.

Note that merely sanding with coarse-grit paper will not be as efficient as a grinding wheel or disk grinder.

Flush with water, and dry

First wash the open blister with water to remove any loose particles from the laminate. Then, prepare a scrubbing solution using a quarter cup of trisodium phosphate and hot water. Use that to scrub the hull clean in order to leach out any remaining chemical liquids. 

Thoroughly rinse the blisters and leave to dry completely. You can use a moisture meter to check, but many experts recommend a two-to-three month drying period for a blister-ridden hull. However, if you’re only dealing with a handful of blisters, two days should be sufficient.

Fill the blister with epoxy resin

Prior to filling, dampen a clean cloth with acetone and use it to briskly scrub each depression. Mix one pump of epoxy, and paint the resin into each cavity with an acid brush, making sure to wet the entire surface. After about half an hour, the application should begin to kick. 

Minor blister filling

For a minor blister, use colloidal silica to thicken a small amount of fresh epoxy to the consistency of peanut butter. You should use this thickened resin to fill the depression completely. A deck squeegee should be used to compress and fair the filler as well as possible while it is wet.

Deep blister filling

If the blister goes beyond the gelcoat, you will need to replace the damaged laminate. Cut out several pieces of fiberglass cloth with increasing diameter; the largest should be the same size as the depression. Make sure to wet the bottom of the depressed area with epoxy before placing the fiberglass disks.

Take care to place the pieces of fiberglass cloth in order from largest to smallest, for maximum adhesion with the original laminate. Saturate the disks with epoxy resin between consecutive layers, while compressing with the end of a brush. Continue adding disks until the repair is even with the contour of the hull.

After every five layers, allow the filler to set. You can then paint over the repair with unthickened epoxy, let it kick, and then repaint.

Sand, scrub and paint

Leave the repair to cure for at least 24 hours, before sanding the entire repair area to level out the filled cavities. This is where overdoing it with the filler mixture can bite back by making you spend a lot of time sanding down the fill. Pro tip: don’t be too liberal with the filler. It saves money and time.

Scrub the entire hull with hot water and soap to remove surface dust and any chemical residue before applying a good coat of bottom paint. 

Wait, no brand new gelcoat?

You don’t need to apply a new gelcoat over the repair job for two reasons. First, epoxy is much more waterproof than gelcoat. Secondly, the whole repair job will be covered with a bottom paint finish, so your boat will still have a glossy exterior. 

As long as you treat blisters early enough your boat will not suffer any structural issues. Ignoring blisters for too long might cause the hull to become saturated – this is a much more serious condition known as boat pox, and requires specialist attention.

Preventing Blisters

To prevent a boat blister from forming, you need a completely waterproof boat. Although this is possible with a complete barrier coating of epoxy, it can be quite expensive. Regular maintenance on your boat is much more cost effective and helps you treat blisters once they appear.

However, caring for your vessel doesn’t have to be a hassle. At Bilbo’s Marine, we do not believe there is any boat too small or any yacht too big to maintain. Contact us today for your boat to retain what we call #thatBilboShine.

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