This discussion of how best to clean gunk off the gel coat on a boat has brought up some interesting questions about different cleaners and scrubbers. The basic conclusion is that pretty much anything that will do a good job of cleaning will also affect the gel coat. Your best bet is to apply a good coat of wax that becomes the layer that traps the gunk before it reaches the gel coat. You then wash off the wax with the gunk and re-wax. If more is needed I suggest a neutral cleaner and baking soda.
As a science nerd with a degree in chemistry, here is what I found:
1. As mentioned previously, oxalic acid will remove rust stains but will degrade gel coat.
2. Bleach will de-color organic compounds such as algal scum but alters the surface chemistry of gel coat.
3. Another "bleaching" compound is peroxide but it also has an impact on the surface chemistry of gel coat.
4. Vinegar is often suggested as a cleaner with a lower impact than bleach or peroxide, but being an acid needs to be removed quickly before it has a chance to react with the gel coat.
5. I have tried using a melamine sponge with success but is the abrasive hard enough to affect the gel coat? The answer to that is maybe. The hardness of melamine is measured on the mohs scale for minerals and is 4. A mineral with a higher mohs # will scratch one with a lower number. Unfortunately, the hardness of gel coats is measured on the Barcol scale (a deformation test) for plastics (~50) and is not easily transferable to the mohs scale. The only comparison I could find is that the scratch resistance of gel coat falls between that of a finger nail (2.5 mohs) and glass (5.5 mohs). (A fingernail will not scratch gel coat, but glass will.) Abrasives with a low Mohs scale rating include baking soda (2.5) and walnut shells (3.5). These might work better for cleaning gel coat.
6. Rubbing and polishing compounds have abrasives with very high mohs (aluminun oxide, silicone carbide, or silica). These should be used only if you want to remove the outer layer of the gel coat.
I think I will start using the Star Brite deck cleaner with baking soda as an abrasive.
As a science nerd with a degree in chemistry, here is what I found:
1. As mentioned previously, oxalic acid will remove rust stains but will degrade gel coat.
2. Bleach will de-color organic compounds such as algal scum but alters the surface chemistry of gel coat.
3. Another "bleaching" compound is peroxide but it also has an impact on the surface chemistry of gel coat.
4. Vinegar is often suggested as a cleaner with a lower impact than bleach or peroxide, but being an acid needs to be removed quickly before it has a chance to react with the gel coat.
5. I have tried using a melamine sponge with success but is the abrasive hard enough to affect the gel coat? The answer to that is maybe. The hardness of melamine is measured on the mohs scale for minerals and is 4. A mineral with a higher mohs # will scratch one with a lower number. Unfortunately, the hardness of gel coats is measured on the Barcol scale (a deformation test) for plastics (~50) and is not easily transferable to the mohs scale. The only comparison I could find is that the scratch resistance of gel coat falls between that of a finger nail (2.5 mohs) and glass (5.5 mohs). (A fingernail will not scratch gel coat, but glass will.) Abrasives with a low Mohs scale rating include baking soda (2.5) and walnut shells (3.5). These might work better for cleaning gel coat.
6. Rubbing and polishing compounds have abrasives with very high mohs (aluminun oxide, silicone carbide, or silica). These should be used only if you want to remove the outer layer of the gel coat.
I think I will start using the Star Brite deck cleaner with baking soda as an abrasive.
