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All chemical bottom cleaners will remove some bottom paint, some more than others, depending on the chemical composition of the cleaner and the paint. Sanding and pressure washing also removes some paint and if it's ablative, just sitting in the water causes erosion of the paint.
The fact that you have some algae on the surface is an indicator that your bottom paint has lost some of its effectiveness and either a new surface needs to be exposed or the paint replaced. A common occurrence is that the active ingredient in the paint, cuprous oxide which is highly toxic to marine life, reacts with elements in the water and becomes copper sulfate at the surface, which is no longer toxic and allows marine growth to attach. The other possibility is that the toxins in the paint have exhausted and the paint is no longer effective. Only the old binder remains which does nothing except slow your boat down.
Building up layers of paint, new on top of old, is a false economy as only the surface is active. Anti fouling paint is very heavy because the toxic biocides in the paint is a heavy metal, and accumulated layers only add to the weight of the boat and the roughness of the surface, both of which slow down the boat and require more energy or fuel to drive it.
It's a good idea to sand the surface each year to expose fresh biocide or chemically clean the bottom to remove remaining marine growth and expose a new active surface. Paint manufacturers will not recommend this practice as they prefer to sell you more paint. I read an estimate that approximately 50% of antifouling paint sold is never actively used but simply covered over with new layers of paint because the boat owners did not know that they could re-activate the paint by removing the sulfated surface.
Be aware that there may be restriction to this practice in your area or it may even be illegal The release of the toxins into the atmosphere from sanding and into the ground, are not environment friendly practices. When removing paint you should use a ground sheet (plastic sheet) to catch the effluent for proper disposal. If you dry sand or soda blast, the proper practice is to enclose the boat in a tent to prevent the release of the sanding dust into the atmosphere. If building a temporary tent over the boat is impractical, wet sanding instead of dry will reduce the release of dust into the atmosphere and the effluent will fall onto the drop sheet Also be sure to protect yourself as the toxic biocides can enter your body through your lungs, eyes even through your skin. These biocides can attack your nervous system.
When the paint becomes exhausted, it's advisable to remove the old binder before applying fresh paint. The exhausted binder serves no purpose.
Another common misconception is that anti-fouling paints will protect fiberglass boat bottoms against water migration and osmosis blisters or boat pox. They will not. You should apply a barrier coat prior to the anti-fouling paint for proper protection.
Aurora Marine Industries are the inventors of chemical bottom cleaners and manufacturers a number of specialty cleaners for different applications: Algae Strip
and Waterline Stain Remover
are industrial strength and are made for fiberglass boats. Inflatable Bottom Spray
is made especially for safely cleaning inflatables, without damaging the fabrics. Algex
is made for aluminum and painted boats but can also be used on fiberglass. It is not as aggressive as the other cleaners, takes longer to work and is not as damaging to the paint. The newest bottom cleaner is Green Genie
which uses an organic salt instead of acid. It is more environment safe, is non fuming and takes a little longer to work but is still an aggressive cleaner made especially for fiberglass. One of these cleaners may serve your purpose.
You may also consider eliminating the toxic bottom paint altogether, as many boat owners are doing, and going to a more environment friendly alternative. There are numerous paints on the market that work by providing a very slick surface that makes attachment of marine live difficult and provides an easy to clean surface. Aurora manufactures VS721
which is not a paint but a clear polymer that is applied annually like a polish. It is much less expensive than paint, easy and safe to apply, will not harm the environment or humans, will also seal the porosity in the gelcoat to protect against osmosis and is proven to provide performance benefits such as better speed, handling and reduced fuel consumption.
I hope this helps you make the right decision for your application.
Thanks for your question, Captain Aurora