Author Topic: Bottom Paint Preparation  (Read 3552 times)

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Re: Bottom Paint Preparation
« Reply #6 on: 16 March, 2011, 10:05:14 PM »
Hi Galeacw - I suspect you got osmosis and you must treat it otherwise the hull will continue to get damaged.

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Re: Bottom Paint Preparation
« Reply #5 on: 26 February, 2011, 07:46:04 PM »
my speedboat is in a basment garage for the winter period and i just noticed that on the decking there are a lot of blisters.  When i pressed them they burst and brown liquid came out !! what is this and how can i cure it ? 


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Re: Bottom Paint Preparation
« Reply #4 on: 02 June, 2008, 08:55:06 PM »
Well everyone seems to reckon on definately doing an epoxy barrier nowadays to prevent osmosis blistering. This seems to be especially relevant on resonably priced mass produced hulls that come from places like Italy, Turkey and the former Eastern bloc where perhaps the quality of the gel coat is not as good as those used on better quality boats in the US.

I checked on a large US forum and they all reckon on putting the epoxy barrier. The last thing you want is osmosis, a nightmare to sort out properly and potentially a boat changer.


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Re: Bottom Paint Preparation
« Reply #3 on: 02 June, 2008, 07:10:05 PM »
Hi Skip,

In my humble opinion, if one uses the boat for the summer period only (3 to 4 months) I don't beleive that the gelshield is really that important. Or am I wrong?



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Re: Bottom Paint Preparation
« Reply #2 on: 02 June, 2008, 05:51:15 PM »
For new boat/hull preparation the general consensus seems to be applying an anti-osmosis epoxy barrier coat first. The two most common products are International's Gel Shield 200 and West Systems Epoxy. Neither come cheap and on an 18.5 foot boat you're looking around
« Last Edit: 02 June, 2008, 08:53:11 PM by skip »


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Bottom Paint Preparation
« Reply #1 on: 29 May, 2008, 09:49:18 PM »
 Let's be honest, applying antifouling paint on the bottom of a boat is not one of life's great pleasures. But a little knowledge of these paints coupled with the right equipment can make the job a lot easier.

Antifouling paints operate on a very simple idea. They simply poison any marine growth that attempts to adhere to your boat's bottom. Chemicals known as biocides do the dirty work. Traditional bottom paints use copper or copper-bearing compounds. Newer formulas use compounds containing tin.

Both copper and tin biocides provide excellent antifouling properties. In choosing an antifouling paint the biocide is probably less important than other considerations such as the colors available or the type of paint already on the boat.

However, copper-based paints must never be used on aluminum boats or the lower units of outboard motors or I-O units which are made out of aluminum. When immersed in water, the copper in the paint and the aluminum set up a chemical battery causing electrolysis which can destroy the aluminum.

In conventional paints, the biocide is mixed with an oil, epoxy or vinyl-based binder. The binder does two things. Most importantly, it sticks the biocide to the hull and keeps it stuck there during the lifespan of the paint. The second function is purely cosmetic. The binder provides the color, except in traditional copper bronze paints where actual powdered metal gives the characteristic copper shade.

Recently, a number of manufacturers have begun offering multi- year antifouling paints. Generally, these paints are known as copolymers due to their chemical makeup. In these products, the binder and the biocide are combined into one copolymer molecule. Although far more expensive than conventional paints, copolymers don't wear out after only one season's use. They remain effective as long as any paint remains on the hull.

Conventional bottom paints have to be renewed every season because of their chemical nature. The biocide leaches out of the binder over time. By the end of a typical season, most of the biocide is gone from the paint. The binder may still retain its original color, but the paint may no longer be providing any antifouling action.

You can't judge conventional bottom paints by their appearance. Color is not an indication of the effectiveness of the biocide remaining. The only way to be sure you have active biocide is to apply new paint every spring.

Look in any marina store and you'll see a bewildering array of antifouling paints. Marketing pressures are only one reason for the variety of formulas available. Experience has shown that no single type of antifouling paint works well under every set of circumstances.

For instance, the western end of Lake Erie where I keep my boat achieves tropical water temperatures in July and August. A lake temperature of 76 degrees isn't unusual. Paints designed for cold, fresh water simply don't work. We need tropical formula compounds.

The same conditions do not exist on the deeper, colder upper Great Lakes where other formula antifouling paints work better. Cold, nearly pure fresh water does not allow typical biocides to work correctly. At least one manufacturer recognizes this and offers an "inland formula" paint designed for these conditions.

River boaters who have tried this paint tell me it simply doesn't work on the Ohio or Tennessee Rivers. Once again, the warm temperatures coupled with farm runoff chemicals are to blame.

Ask around your harbor to find out what type of paint seems to be working best. Get a variety of opinions before making up your mind. Be prepared to switch brands.

Removing all the old bottom paint is not necessary every season. You can paint over well-adhered, smooth old paint. All chips and scrapes in the old paint should be sanded smooth to prevent rough spots which could cause drag. A rough bottom can prevent sailboats from achieving maximum speed or cause powerboats to burn excessive fuel.

It's a good idea to sand back the old paint between coats to prevent too much paint buildup. A thick layer of paint is invariably a rough layer. Also, thick layers tend to peel away from the hull, especially on planing hull powerboats.

When you sand, remember that the biocides in bottom paints are poisonous. Always wear a dust mask and eye protection. Gloves and long sleeves are also a good idea since some of the biocides can be absorbed through your skin.

In choosing a new bottom paint, you must pick one that is compatible with last season's paint already on the boat. Some binders will soften and lift old bottom paint much like a paint remover. The result is total mess which has to be completely removed before starting all over again from a bare bottom.

How do you know if new paint is compatible with the old? First, read the label. Paint manufacturers go to great pains to save you from making mistakes. Labels provide a wealth of information, so read them.

If you are still not certain, apply a small amount of the new paint over a test patch of the old. Be sure the test area is normally not seen when the boat is in the water. Obviously, keep the test patch small just in case the new paint isn't compatible with the old. You don't want to create any more work than necessary.

Never try to stretch bottom paint by excessive thinning. You're just kidding yourself. Sure, the bottom will look good, but the layer of biocide will be too thin to last the entire season. By August you'll have a garden growing around the waterline. Once again, read the label before thinning and never use more thinner than recommended.

If you do thin the paint, be sure to use only the exact thinner recommended by the manufacturer. antifouling paints are complex mixtures. Using the wrong thinner can turn them into useless taffy or destroy their antifouling properties. This is one time it's smart to spend a couple extra buck on a brand-name thinner rather than a cheaper, generic product.

Bottom paint can be applied with a brush or roller. Leave spraying to the professionals. Antifouling paint never seems to clean out of brushes, so the disposable kind make sense. Be sure to get bristle disposable brushes. The ones made out of foam simply fall apart long before the job is done.

A long handle on the roller will allow you to work standing upright beside the boat. You won't have to crawl on your back in the mud to reach the keel. The best handle extension is made in a single piece of wood about five feet long. It screws into the plastic roller handle.

Professional painters know that small rollers only two or three inches wide can really speed up a job. These rollers aren't generally available in marina stores. They can be found in regular paint stores where the pros do their shopping.

Plastic roller tray liners make cleanup a snap. I've always used them without trouble. But readers have reported that the solvents in some bottom paints have softened the plastic used in pan liners. Once again, a test with a small amount of paint is in order. If in doubt, line the roller pan with aluminum foil.

Wear gloves, long sleeves and eye protection. Get as little of the antifouling paint on your skin as possible. What paint does get on your hands should be cleaned off immediately in accordance with the paint manufacturer's instructions.

Masking tape seldom cuts a smooth line at the boot stripe. Bottom paint has a tendency to wick under ordinary masking tape, giving a rough finished line. Although it is more expensive, special "fine line" tape is available. It seals tightly against the hull and provides a clean line between the paint and the boot stripe.

Boot strips are generally molded into the hull gel coat. On many boats, some of the strip is under water. Invariably, it gets some fouling, yet it cannot be painted without ruining some of the graphic look of the boat.


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