Fiberglass Boat Repair: 
Gelcoat Blistering

No, that isn't a case of Boat Chicken Pox, but what it is is a case of blistering.  Where did they come from? What can be done?

There are several reasons for blistering to occur on your boat.  But there is one main underlying reason... cheap resins!  As you stand and muse at the pock marks all over your hull, look around you and notice other boats that may be up in dry dock.  More than likely, you will notice some with blisters and some without. 

In my studies of laminate and top coats and in my observations over the years in working on them, the answer has come clear, there are a few different reasons for blistering. Grades of resins which boat manufacturers use in the building of their boats is one.  If a manufacturer uses a cheaper grade of resin on the boat, it will likely encounter blistering at some point. 
Another reason would be poor workmanship in the laying up of the fiberglass layers. 
Below are a few of the more common reasons.

Cheap Resins

If a boat manufacturer uses a cheaper grade of resin in the manufacturing of the boat, i.e. one that is not all together not water permeable, it is only a matter of time before those blisters show up.  No, it wasn't the marina's fault.  It was the foundation of the boat hull. 

Too Much Hardener in the Gelcoat

If the boat manufacturer put a bit too much hardener in the resin mix this will also cause blistering.  But these blisters are going to show up before the boat ever leaves the manufacturers yard.  This is because hardners mixed with the resins cause the mixture to heat up.  The more harder that is added the hotter the mixture becomes.  Many times a boat hull will develop blisters right in the mold, as the gelcoat bubbles and blisters. 

Too Much Hardener in the Fiberglass

If the manufacturer used too much hardener in the fiberglass itself, some problems could occur that will lay the path for the gelcoat to later blister or incur stress cracks. Fiberglass is a little bit more forgiving than gelcoat where too much hardener is concerned.  None the less, it is not a foundation which is desirable in the building of a boat.  Too much hardener in the fiberglass will cause the fiberglass resin to have it's strength compromised, or even to become brittle and to crack. Then when the gelcoat is applied, it lays on top of a surface which will later break down with flexing and movement. 

Not Enough Wetting

This is another case which will cause the blisters to show up at some later time, more than likely.  When the manufacturer "lays up" the fiberglass of the hull, if they don't use enough resin to really soak that fiberglass mat down well, it leaves some dry areas of fiberglass mat.  Fiberglass mat is composed of fiberglass shards all woven or pressed together. If wetted properly, the mat will become soaked with resin and not only soaked through, but will also have a smooth layer of resin (or what appears to be) over the top of the fiberglass as well.  This makes a nice foundation for the application of the gelcoat finish.  But with dry areas this is not the case.  With dry areas of matting, all is lost. There is no soaking, no smoothness.  When gelcoat is applied, though it may not be very visible at the time, the gelcoat lies on top of a somewhat dry peace of cloth mat.  The gelcoat itself will try to bond with that, but it won't be able to hold it's own.  It needed the help of the sub layers of laminents. Little bonding will take place, if any at all, between the fiberglass layer and the gelcoat.  This in turn, will allow the gelcoat to separate from the fiberglass at that particular spot. 

The Void

No, this isn't like a black hole in outer space were talking about, but it is what occurs when fiberglass is not pressed and saturated properly in the manufacturing process of the boat. What happens is this: The fiberglass mat, if it is not pressed or rolled enough to the point where it is all smooth, and if scuff sanding is not done thoroughly, a hull can wind up with little low spots, valley's or ditches in it's surface. The surface not being completely flat, even if it is a very small divot barely viewable by the eye, can cause major problems for the boat surface. If premium resins were not used, then scuff sanding is necessary to help the gelcoat to bond with the fiberglass. If there are smooth areas on the fiberglass (i.e. shiny) because of low spots that were not scuffed, the gelcoat may not bond well, if at all. Again, when the boat goes pounding across the water, the weak, unbonded gelcoat surface gives way. Voids by the way, can occur anywhere on the boat, even on the upper decks and the bridge.

What can cause blisters to show up later on?

Depending upon which error the manufacturer made, blisters don't always show up immediately. Sometimes they just show up unexpectedly.  If the layers of fiberglass mat were dry in some areas, when the boat pounds across the water, and because the gelcoat was not bonded well,  or at all with the fiberglass, the gelcoat may become weakened and it will flex.  This can in turn cause a tiny little stress crack to appear on that area of gelcoat. The small stress crack then allows water to be forced in and under the surface of the gel, which in turn causes a blister to appear.  Pressure washing can have the same effect. But again, it is not the fault of the marina or the guy who does the pressure wash for you.  If the foundational structure and the surface of the hull were what they should be, then pressure washing will cause no problems for your boat.
If the fiberglass had too much hardener put in it during the manufacturing process, when the boat then pounds across the water, because the fiberglass strength was compromised, it can shatter because it was brittle.  When the fiberglass shatters underneath, the gelcoat has no choice but to crack or to blister.

What must be done?

There are several methods to take care of the blistering problem.  All of them can be costly. Depending upon how you want to approach your blistering problem, effects the outcome of the problem solving.  Are there just a few blisters?  Is your hull covered with them?  Do you want to try to rectify the problem for good? Do you want to spend a little money or a lot of money? 

The Approaches

Yes there are several ways you can go about maintaining a boat that has blister problems. Of course there is only one way this should be done to stave the problem off for good, but this is not always what the boat owner wants to do, or has to do.

A few blisters here and there

If you have only a few blisters, they can be taken care of individually as they come. Many boat owners prefer to do it this way because it can be less costly for them if they don't plan on keeping that boat for many more years.  I have seen some boat owners not even care to fix their blistering at all, but I must assume that if you have read this far, it is because you most likely want to do something about it.  You can, if you wish, have each blister fixed individually.  The process involves grinding each blister down to the good fiberglass underneath, and fresh new glass being put in and then gelcoat applied.  This is the best way for you to tackle a few blisters here and there without spending a ton of money to have your entire bottom done.  Typically, blisters can show up here and there each year.  It's a question of whether you want to spend a little each year, or if you want to spend a lot at once. But know this, that even if you think you are going to get out of the blistering problem simply, by this method, you can be deceived! If the fiberglass mat underneath the surface is a mess, and there are millions of white spots and voids or unsaturated fibers, all of which are going to conduct water via the capillary effect, then the fiberglass will have to be ground back and down to a point where good glass can be found.  And sometimes this can reveal a very large surface area!

Absolutely covered with blisters!

If the hull is just covered with the blisters, and fixing individual blisters here and there is not an option then you will have to consider fixing the entire bottom. 

Your options:

Cosmetic
If you just want to do a cosmetic repair for now you can have the entire bottom sanded down and filled with plastic fillers, flat sanded and bottom painted.  This will cosmetically fix the problem for a period of time.

A Real Fix (hopefully)
If you want to fix the problem for real, then the gelcoat surface will need to be completely removed, ground down, laid up with new fiberglass and re -gelcoated.  At this point, you and your maintenance people are at the mercy of the quality of product which is used in the repair of the boat.  The person who fixes the boat can buy the absolute best materials available and if the manufacturer of the resins themselves have botched a batch of resin the problem can occur again. If materials were used that were known to be top quality, but yet there was some problem in the manufacturing of that product batch, and IF the person fixing your boat does a good and complete job, the problem can still be staved off. 

Your best bet 
When you buy a new boat have an inspector inspect the hull meticulously. If a blister shows up during the time of warranty, be sure to take action while it still is.  Though sadly, and I have seen this happen all too often, boat manufacturers will tell you to limit the time your boat is in the water to a couple of weeks at a time! I am not a lawyer and cannot get into the legal aspects of this topic but I can tell you this, you may have a court battle on your hands in trying to get a manufacturer to make right on the boat. If you are buying a used boat, again, have it inspected by a professional inspector. Maybe it has some blisters on it already that can be seen.  If this is the case, you may consider looking at a different boat. Another possibility is that the owner of the used boat you are looking at has recently had some cosmetic work done on the boat to hide a problem of blistering. Take note and proceed with caution if a boat has a fresh coat of bottom paint on it, if if it has been painted or gelcoated recently.  If a good body man has done the work, it will take a highly trained eye to perceive the work that was done.