Repairing blisters on your fiberglass boat bottom?Permanent repair with epoxy filler is the best fix and here's how!

Your life may depend on what's below her water line.

If you own a fiberglass boat that stays in the water year round,you may be susceptible to blistering of your bottom[the boat bottom that is...yours won't happen till you pay the bill]. I spent two weeks repairing various sized blisters on Nomads[my love and my lady] bottom.My research led me to a mythology almost as involved as the Greek's!

I've sorted through the facts about causes,severity,and permanent repair with the help of a master fiberglass repair technician[thanks Pete]. I paid him,you get the info free!'s the strait poop.

Older boats are more prone to blisters due to aging gel coat which becomes permeable[allows water to pass through it] with age.New boat owners should not be complacent however, because nicks and dings from every day use will start the process of blistering as well.The point is;Your gel coat is not just there to look pretty.The Gel coat acts as the sealing final coat which keeps sea water from being absorbed by the structural fiber glass beneath it.

What is a Blister?

Nomad had two types of blister's on her bottom and sides:Osmotic blister's which are filled with a toxic mixture of chemicals and sea water,and Void surface blister's which are the result of poor bonding or chemical failure etc[more on both of these latter].

Conditions during lay up of the hull can predispose a boat to blisters.Construction facilities should have vacuum hoses attached to All tools and the building should be kept at a constant Low humidity with air filtration at All times.In a perfect world,that's how all boats would be built.In our world,it ain't so.Dust particles in the air that settle on the hull as it's being laid up or sprayed create tiny pockets within the glass layers which immediately draw excess catalyst into them;That's one reason the catalyst to resin ratio is so critical during lay up.When the gel coat fails,or is damaged,osmosis begins.Osmosis is the process of drawing water into the blister in order to equalize the catalyst solute with the exterior water.The process of Osmosis makes the blisters larger.These are the blisters that will grow large enough to compromise the integrity of the hull.

Two types of resin you need to be familiar with.

Polyester resin is most likely the resin used to build your boat.Polyester is a good general resin for boat building...and it's economical...but you get what you pay for.Epoxy is the preferred resin because of it's incredible strength[seven to ten times that of polyester]and it's amazing molecular density which makes it almost bulletproof and impervious to water absorption.You should never lay Polyester over Epoxy...that's a no no.You can however,lay Epoxy over Polyester...That's a very good thing.

The two common methods of building hulls.

The two most common methods of fiberglass boat building are hand lay up and chopper gun spray.The preferred,though more expensive method,is to cover the hull mold with alternating layers of Matt,roving,and cloth fibers which are then saturated with a resin catalyst mixture,also by hand.Built in the right environment this yields a strong consistant hull.

The second method is to spray a mixture of chopped glass fibers and Polyester resin.This is called chopper gun because the spray gun continuously chops long strands of fiber and mixes them with activated resin as the whole mess leaves the gun nozzle and is sprayed onto the hull mold.The end result is mufti directional fibers.Look Inside your hull to determine how your boat was built.If your boat was built this way I strongly urge you to consider another boat.Why?

Because chopper gun does not lay down a uniform density of glass fibers so your hull will have differing densities that leave weak spots.That's why some of these hulls will hole in heavy weather when struck by a sharp wave...but it is a cheap fast way to build a hull.In all fairness,It Is possible to Spray a 'Healthy' hull.However,the skill of the gun operator is crucial and very few operators are able to take the time in an assembly line boat factory to insure uniform fiber density over the entire surface of the mold.Hand layup is preferable to chopper gun unless you're in protected water.

As I've said,Epoxy is far and away superior to Polyester resin but can cost many many times more.A hull built with Epoxy would cost several times that of one built with Polyester making the luxury impractical for most.There is a compromise that works beautifully however buy filling and sealing blisters and hull with an Epoxy filler and sealing coat.That's where blister repair can be made Permanent.

Doing the Dirty Deed.

First you must haul your boat and pressure wash her bottom.DO NOT get closer than ten to twelve inches from the hull with the washer wand.Some high pressure washers have such a high psi rating they can actually create micro-fissures and exacerbate the problem.

Finding the Blisters

The cause of blisters is irrelevant to this hub,it's the Permanent repair we're concerned with.A good analogy to blistering on your bottom is the same as a case of acne[kinda weird but stay with me here]acne is never uniform.Zits come in all sizes and depths.Bottom blisters are exactly the same and detecting them is not as easy as you might think.

I must stress that working fiberglass requires a gentle hand so don't force the next step.

When you've removed the sea gunk and your certain your lady's bottom is absolutely dry, it's time to lightly sand with Medium grit[preferably metal based]paper.As you Lightly sand her bottom the surface blisters will open up.Usually there is a little liquid in these,try not to get it on your Hurts! Be sure you blend the edges where the blisters have ruptured so you end up with a slight indentation not a crater.Take your time here because any missed blisters Will be sealed into your hull and draw water from your bilge...that's a bad thing..

Now comes the tricky part but I know you can do it.

Dangerous blisters will eventually absorb enough water to be reduced to the consistency of tissue paper.Any deep blister that goes untreated will eventually cause De-lamination which requires Expert removal and hand lay up to maintain hull integrity.These deep blisters are rarely visible because they spread out under the layers of glass they've infiltrated.

Here's how to find them...and it'll burn you out a bit so get some beer for your cool down.

Get yourself a flat faced hammer,the type doesn't matter,what ever goes with your outfit will due.TAKE Your time and Gently sound her hull,tapping all over her bottom.Think of it the same way you do when your thumping a melon.Your listening for the same healthy hollow sound.If you hit any dead or muffled spots circle them with a permanent marker.When you've finished this sounding your brain will be exhausted so have a few beers and call it a day.The next step is critical and you want to be absolutely Fresh for it.

Drilling for the nasty catalyst.

OK,when you sober up,get a variable speed drill with a quarter inch flat tipped bit and your ready.First we'll identify flat spots that are there because they are backed by something Inside the hull like mounting brackets for bulkheads,engines,tanks etc.This step requires two people,one inside and one out side the hull.Two way radios will come in handy here if you want to avoid a lot of shouting.One of you taps the circled dead spots,the other identifies the healthy dead can mark these with a different colored marker so you don't drill into them.

Once you've ID"d the healthy dead spots,the rest are the deep down pimples in our analogy.Tap around until you find the center of each blister then drill Slowly into it.Do not get your face close.Stay at least a foot or more away because the big ones are usually under pressure and when you hit the liquid center they Will erupt spraying you with a very dangerous caustic mixture of catalyst and water.If they don't contain liquid and you find a void instead,that is delamination or was there from construction but don't lose heart,we'll treat it just as we would a blister.Let the Blisters drain until they're dry then get your disk sander and remove All of the glass layers covering them until you have a gently sloping indentation of Healthy glass.Be sure you blend or feather the edges so you have a depression not a crater.

Scrub her bottom

Now is the time to mix some phosphate,according to the instructions on your preference,with warm water and Scrub her bottom.Put your heart in it....Do it again and again until there's no trace of the nasty juice.Scrubbing with a clean brush after you rinse is a good idea.You'll see raw glass where you've removed the gel coat and glass layers,It won't be pretty.If her blisters continue to bleed out,you have more sanding to do.When your satisfied that the Poison has completely drained let her dry out for as long as you can.Three or four days in hot weather minimum.

Please keep these bits of wisdom in mind.

1 Epoxy or Polyester may be used over Polyester.Never apply Polyester over Epoxy.It will not grip and your repair will be for not.

2 Always scrub Epoxy after it cures to remove wax that rises to the surface as it cures.

3 substitutions[the cheap way dehydrated lime instead of micro-balloons or aluminum powder]will always bite you on the ass.

4 your Life may depend on this fix one day so Don't experiment with boat yard home"remedies"...and believe me,there are all kinds of strange remedies floating around.

And now for the filling,shaping,and sealing.

OK,her bottoms nice and dry and there's no weeping from any of the open blisters.Some people prefer laying glass fiber in the large blisters but these have a tendency to unplug and I don't really trust this method.The Epoxy filler we're going to make is very tenacious and wont come loose.

The magic stuff that makes her tough.

I used West System Epoxy for my repairs and highly recommend it[I'm not getting paid for this] .The resin and catalyst come in cans with pump nozzles that are proportional[one pump to one pump] so your catalyst to resin ratio is perfect...that's a good thing.You can pick up cans of Aluminum powder from them as well.The strength and durability of the aluminum added to the resin is way stronger than the surrounding glass.

Mix the Aluminum powder into the Epoxy until you have a 'Spackling compound' consistency.Fill all blisters large and small.Try to level the filler as you apply it.Epoxy doesn't shrink like Polyester when it cures and is incredibly dense...and sanding this stuff to level it is almost as much work as having a long conversation with my ex wife[how I miss that girl]. Remember,always scrub and let dry before sanding.Epoxy has wax in it that comes to the surface while it cures and you must remove it.Once cured and scrubbed,Sand her bottom to a Medium grit finish.You're shaping her bottom so take care.Let the artist in you take charge.

The finishing touch.

At this point we need to apply a protective coat or two or three[at least three,six would be best]of Epoxy with a small amount of aluminum powder added to it so the hull is sealed again.Brushing it on is best because the aluminum tends to settle and a roller wont pick it up very well.I Know this is redundant but Wash and very[I Mean VERY] Lightly sand her Bottom between coats.The great thing about this finish is Epoxy is self leveling so her bottom will be smooth and beautiful.Finally,a light sanding so the bottom paint can get a grip...which is what I'm gonna do right now on a few Heineken's.

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vrbmft profile image

vrbmft 6 years ago from Yucaipa, California

Whoa! I don't have a boat. I have a bottom, but not a boat bottom and I wouldn't consider doing anything of that to the bottom I have. But if I had a boat, I might be tempted to do the deed with your hub in front of me and of course beer in the frig. Wow! So you are one of those folks who have that knack. Are you a tactile learner? Your eyeballs are in your finger tips at least for projects like this?

Reading this reminds me a little bit of repairing shoes back in my college days and sanding the excess sole was very important, tricky and a real skill that I developed over time. In the mean time, I effed up a few pair of otherwise perfectly good shoes. Told the guys it was a new style and pretty soon everyone would wear shoes that looked like there's (because it would take me a while to get skilled!!!) I repaired those babies for five years and got pretty good at it, even though I am not a tactile learner!! Put my soul into it though and enjoyed it immensely. Yes, that was a pun!

Very very informative hub. Well written and easily followed when and if one has to work on her bottom!! And it won't cost as much as having someone else to do it. It will be a "cheap" bottom job well at least perhaps cheaper, all because of a hubber named cheaptrick. Pretty cool!


Paradise7 profile image

Paradise7 6 years ago from Upstate New York

Terrific and useful hub. Thank you!

GmaGoldie profile image

GmaGoldie 6 years ago from Madison, Wisconsin

I have bookmarked this for future reference, my boat needs painting and you have all bases covered - thank you!

cheaptrick profile image

cheaptrick 6 years ago from the bridge of sighs Author

Hi GG.If you have any questions if and when you do this,don't hesitate to get in touch.Also,I think it's a good Idea with older boats to apply an epoxy sealer coat as a preventative measure,could save you money in the long term.Thanks for reading.

Txoof 2 years ago

Good information, terrible punctuation!

cheaptrick profile image

cheaptrick 2 years ago from the bridge of sighs Author

sorry about the punctuation Txoof.I'll try to fix that as soon as I can.

Trix 22 months ago

I read your posintg and was jealous

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