Olene Boyko of Urbanna, Virginia, asks:
"On removing the inspection plates on the welded steel water tanks aboard my 48–foot boat, I could see they were painted on the inside with a blue high-gloss rubber-like paint. I also saw quite a bit of corrosion around the edges of the opening for the inspection plates and a considerable amount of scum and algae growth. Bleach has taken care of the scum, but there doesn’t seem to be any way to strip off the paint and start over. I can brush and lightly grind the rusty spots, but I’m not sure what to do next. I would like to repaint the rusted areas with a water-potable paint and have looked at several paints with rust prevention characteristics, but I am concerned about their compatibility with drinking water. Unfortunately, no one seems to know what paint is inside the tanks and the builder can’t be reached for advice."
Don Casey replies:
Believing that an inert paint product would be able to prevent or retard algae growth was a bad idea right from the start. The usual way to fight algae is to add a small amount of chlorine to the water. If the painted surfaces have protected the interior metal from corrosion and the tanks still are intact after 18 years, except around the edges of the inspection plates, I would leave the paint undisturbed. My guess is that if the corrosion is limited to the area around the inspection ports, the probable culprit is acid in the silicone sealer that was used to seal them.
The standard tank coating today is a potable-water approved epoxy formulation. These epoxies are made by a number of manufacturers. An added benefit of epoxy is that compatibility with the blue coating, whatever it is, shouldn’t be a problem. When you use one of these epoxies, overlap the repair onto the original paint to create a good seal. No matter what epoxy coating you choose, follow the instructions carefully, particularly with respect to the primer you use on the bare steel. Also be sure you remove every bit of the silicone sealant before applying any new coating.
As for the inspection plates, if they let any light into the tank’s interior this will encourage algae growth. If the tanks and plates aren’t in a dark location, paint the exterior of the plates so they are opaque.
Finally, because you can’t identify the existing paint inside the tank, I think you should test the water for chemical content. Try to test water that has been in a tank for a period of time.