If you own an older boat and are worried about osmosis problems, there are a number of cures and they do not need to be expensive. If no sign of bubbling fiberglass has shown up yet, there is a pretty good chance that action on your part will prevent it from ever happening. Since boat builders recognized the osmosis problems in the late 1980s, most offshore boats have been built with either a vinylester or epoxy gel coat to help prevent bubbling, but older boats certainly should be protected. The best part of this type of bottom protection is that you can do almost all the work yourself if you don't want to spend a lot of money.
The first step is to get your boat's bottom clean of old paint. This is a project that can be tackled over the winter. If you plan on doing it yourself, you should first find out if the yard where your boat is stored will allow you to do the job. If they will not, you may have to resort to a commercial solution or you may need to bring the boat to a location where you can do the work.
Start by placing a tarp or sheet on the floor around your boat to catch all the goop that will come off. To get the original bottom paint off the boat there are several methods.
The easiest method is to have the boat soda blasted. Professionals take care of all the sealing off of the job site, run their own compressors and do the job in a day or two. They also clean up and remove the mess. When they are done, all that is needed is a light sanding with either a longboard or a random orbit sander and the job is ready to be painted.
The next easiest is to bring in your crew and hand them sanders, either longboards (for racing boats) or random orbit sanders (for cruising boats). You will need a power supply, respirators and coveralls, but a crew of five could manage the job in one to five days depending on how much paint is on the bottom and how large the boat is.
The messiest way is to use a paint stripper such as Interlux's Interstrip. You slop the glop onto the hull, wait a short time and peel off the old paint. You will need a Tyvek suit, rubber gloves, and a face mask (the stripper does nasty things to skin). When the stripping is done the hull is washed down and sanded to get the last vestiges off.
With a hot gun, there is no guarantee that the paint will melt and come off, nor is there a guarantee that you won't burn your boat to the ground. That said, a heat gun should be able to soften the paint so that it can be removed.
Now that the boat has been stripped it is time ensure the hull is dry. If it is not dry, you may have to put heaters around the boat to dry it before applying the barrier coat. A moisture meter can tell you how dry the hull is and most surveyors can do the job in a couple of hours. Drying can take a week to six weeks depending on how wet the hull is. If you neglect the drying and coat the hull, you could trap water inside the laminate and still see bubbles later on.
When it comes to barrier coat manufacturers and their products, in most cases, you should use the product made by the bottom paint manufacturer that you intend to use. Bottom paint manufacturers have tested their product against their own bottom paints and know that the two are compatible. Some pure epoxies should not be used for bottom coating because they may turn brittle during the winter months in the boatyard and crack or they may crack when your boat flexes in the water.
When applying an epoxy bottom coat you are looking to build up a layer that is 10 to 12 mils in thickness. (Some epoxy bottom paint makers provide a gauge to enable you to see how this your paint layer is. If you have a gauge, use it.) In some cases this means applying two or three coats to get the required thickness. The epoxy bottom coat can be sprayed on, although it takes more coats to build up the right thickness. The most common way to apply an epoxy bottom coat is to roll it on with a 3/16" sponge roller and "tip" it off with a brush.
How to "Roll and Tip"
Apply the paint with a roller and work over an area about a yard square. Immediately, use a three or four inch dry brush and using just the tip of the brush, smooth out the paint layer. If you work horizontally, the brushed paint lines will gradually smooth themselves out as they sink toward the lower lines. Work up another area and tip it off using the brush. You will find exactly how far you can progress before having to come back and tip off the paint layer.
At no time should you leave a dry edge to the paint layer, you will never be able to fair the edge in properly and will have to sand it off to ensure you get a good edge between paint layers.
With certain epoxies you have to apply a second layer within the paint window. Make sure you have read the instructions carefully and apply your second and third coats inside that window. If not the bottom will have to be sanded lightly and a third coat applied on top of the two layers already applied.
Our Project Boat
Our project boat, Stryder is a 47 foot Bristol built in 1986. The boat has never been protected against osmosis, but fortunately it has been used only for a short period during the summer and left on the hard for much of the year. This means that the hull is very dry and does not need further drying.
The hull was first stripped of old paint when it was soda blasted by Salty Dawg Boatworks. This removed ten years worth of bottom paint in two days. All the masking off, covering of the work site with polyethylene sheeting and a thorough cleanup was done by Salty Dawg. With the hull soda blasted clean, painter Keston Smith sanded any residues away and applied two coats of Eco-Clad Protector 848. The bottom paint was to be Eco-Clad Fast Coat antifouling and he wanted the barrier coat and the bottom paint to be compatible. The barrier coat dried to the was sanded lightly before a third coat was applied shortly before the boat was to be launched.
Photos by Roger Marshall