Two more cents
( a highly simplified explanation )
If I knew what an osmosis test was I could be doing a lot more business as I get asked for it all the time. Dock talkers often use "blisters" and "osmosis" as interchangeable terms however, there are different types, causes and degrees of blisters and using the word "osmosis" just confuses the issue. I am often asked if I can perform an osmosis test to which my response is " What's an osmosis test ?". This response is met with a blank look as the people asking the question really don't know what they are asking.
This is one of the reasons I dislike the word "osmosis" as applied to boats, it has become a generic term to describe all types of blisters and moisture content in fiberglass hulls cored and uncored. Strictly speaking for osmosis to occur you need fluid on both sides of a membrane. If you have this situation on a boat, you have more problems than a few blisters. What you are really encountering is simply water soaking into a porous material. Polyester, epoxy and vinylester resins are not waterproof, they are hygroscopic ie. They can absorb and retain water! (less so with epoxy and vinylester). I have heard many convoluted definitions of osmosis in attemps to justify the use of the word but what we really have is simply a process of absorption.
OSMOSIS: The tendency of fluid substances, if separated by a porous, membrane to filter through it and become equally diffused.
So lets forget about the semantics and get to the issue................
There are many of causes of blistering, To list a few ....... trapped moisture during moulding, undercured resin, overcured resin, aerated resin, incorrect timing of subsequent layers, absorbent fillers, voids, trapped moisture in core materials, stale catalyst, emulsion bound mat, dusty mould, hygroscopic dust, cold mould, inadequately mixed resin, uncontrolled temperature and humidity levels during moulding process, uncontrolled temperature and humidity of raw materials in shipping, orthophtalic (cheap) resins. OK .... enough ! If I really thought about it I am sure I could come up with more but lets just say this, it is an extremely complex issue and "osmosis" just does not cover it. all you need to know is about blisters.
If you insist on calling osmosis then it follows that all fiberglass boats have it !
NO VOIDS = NO BLISTERS (maybe) : There are dozens of reasons for voids in a laminate (some chemical) and they can range from tiny champagne size bubbles to several square feet although most are less than 1/2" in diameter. The average laminate may be 8-15 layers of various types of glass fabric made up of millions of miles of microscopically thin glass strands wetted out with resin. It is unreasonable to assume that all voids will be filled whether the wet-out is accomplished by five guys with rollers or one of the admittedly better (but not perfect) vacuum bag processes. All fiberglass layaups have voids, some more than others. The higher the void content, bigger the voids and the more likely and earlier you will see blisters.
A typical 30' , uncored sailboat hull can absorb about 30lbs. of water or roughly 3% maximum weight of the laminate. The glass fibers do not absorb any water and the resin is chemically incapable of absorbing more than 3% so theoretically 3% water content is sturation point of the material (voids excluded). As all polyester reinforced glass fiber and gelcoats are water permeable to some degree, all fiberglass boats left in water long enough will absorb water and probably develop blisters. This is rarely a structural issue (at least in our lifetime) although it can drive the sailboat racers nuts !
If the hull is cored with balsa or the laminate is all chopped strand (read - very cheap boat) then you may have a more serious and expensive issue to deal with as the balsa rots when wet and in the case of chopped strand fiber, the millions of exposed fiber ends wick water like so many straws.
The more common gelcoats are simply pigmented polyester resin of varying levels of quality and these pigments combined with the aeration caused by spraying the product into the mold can make it more permeable than the resin used in the laminate and therefore most blisters appear in the gelcoat. These blisters are usually small (1/8 - 1/4" dia.) and round in shape. While this does have an effect of the sale value of the vessel it is rarely a cause for concern.
Under the gelcoat is usually a "skinout" mat of chopped strand glass fibers that does not contribute much to the strength of the hull but is used primarily to hide the basket weave pattern of the heavier woven fiber which (hopefully) makes up most of the laminate. When water passes through the gelcoat it may wick up the chopped strand fibers of the "skinout" mat. These blisters are usually small, elongated and again not a major concern.
Water that has passed through the gelcoat and skinout mat into the structural laminate may combine with soluble elements that may occur in the voids. These elements could be uncatalysed resins, silane, glycol or salts (not the table variety) or any of the other chemical soup of ingredients that results from the resin curing (or uncured) process. When water combines with these molecules a new, usually larger molecule forms (hydrolysis), thus preventing escape of the fluid since the molecule is now bigger than the microscopic hole it came in through.
HYDROLYSIS: A chemical process by which the oxygen or hydrogen in water combines with an element or some element of a compound to produce a new compound.
As these newer, larger molecules multiply deep in the laminate they can get big enough to start to pushing apart the various layers of the laminate as the resin dissolves. This can be a serious issue, it is however relatively rare.
Put on a pair of safety goggles and puncture a few blisters. If they are dry or release a clear fluid, you likely do not have the serious type of blister. If an acidic vinegar like fluid appears this could be the more serious "Hydrolysis" type blister. Be careful as some of these blisters contain fluids under tremendous pressure. If the blister is very large and cannot be punctured with an ice-pick, it is likely very deep in the laminate, in this case drilling a hole for closer inspection may be warranted.
THE CURE? Sorry..... regardless of the chemical companies hype there is no cure. You can only delay the inevitable but a very high percentage of blistered boats will still outlast you. I have surveyed one local boat three times over the last 10 years and each time it was getting another $10k "epoxy bottom job". I refused to survey it a fourth time because a well respected shop was about to do it again using the same improper techniques as the previous three jobs done by others. Whoever buys this 42' motor yacht will be doing it again soon (if there is any laminate left).
Small blisters in the gelcoat may be repaired by sanding, drying and applying an epoxy or vinylester bottom coat. This may help the resale value of your boat. Blisters in the skinout mat can be repaired by the same method but with much more aggressive sanding and perhaps some patching. The larger hydrolysis blisters require complete removal of the gelcoat and probably the skinout mat and perhaps a layer of the laminate (perhaps in local areas or over the entire hull in which case new cloth may be needed). The hull must then be dried to the point where the epoxy or vinylester bottom coat will adhere and washed frequently during the drying process to wash off as much of the hydrolytic fluid as possible. This fluid and water may weep from the hull for weeks, months or even years so washing is crucial to providing a clean surface to ensure the adhesion of the new barrier coating.
This can be a bit of a gamble. Many bottom coats fail because the hull was not dried or washed properly. I have seen boats put under heat lamps for six months before bottom coating and new blisters appeared within a few years. The moisture is so deep in the laminate at a molecular level that it is not easily evacuated. A new system of applying heat under vacuum holds promise for drying hulls but for the most part it's still a gamble.
Give this careful consideration before plunking down $5,000.00 - $10,000.00 - $20,000.00 or more for a bottom job and always ask for a written guarantee (unlikely). Consult an Accredited Marine Surveyor® before spending you're hard earned money.
Captain Wallace Gouk AMS®
Port Credit Marine Surveys
Dirt People Scare me