If your boat sits in the water for an extended period of time, it may develop a "scum line" that is difficult to remove.  In this article, I will describe how to remove that scum line easily and without a lot of excess scrubbing.

Please note that I have included links to some products referenced in this article for your convenience.  If you click on a link and buy something, BoaterInput will receive a small commission to help defray some of the cost to maintain this site. 

The Problem

A scum line is simply a discoloration or staining that occurs at the waterline due to organic growth, minerals or other pollutants in the water.  This forms every Spring on my fishing boat after I navigate into shallow, murky backwaters in search of monster bass.  But it happens on my pleasure boat too after a long day of skiing and floating on the lake.  The problem is, besides not looking great, the longer you leave the scum line on there, the more difficult it is to remove.  Therefore, it is important to treat the problem in a timely manner.

Here is a picture of my boat after power washing it and scrubbing with a brush and Dawn dish soap.  Notice that the scum line still appears.

Scum Line on a Boat

The Solution

My neighbor, Mark, told me about a product that is designed just for this purpose and so I decided to give it a try.  It is called Starbright Instant Hull Cleaner and it is available in a liquid or gel (spray bottle) form.  I chose the latter thinking it would be easier to apply.

The process to clean the scum line off your boat is really quite simple.

  • Pre-Clean Your Boat.  Ideally, this should be done with a power washer and some soap (e.g., Dawn dish soap).  However, a garden hose should work too but will require a little more effort.  Doing this pre-cleaning will help avoid using more of the product than is necessary.
  • Spray on the Instant Hull Cleaner.  Apply it to a portion of the scum line and then wait two minutes to give it time to loosen the stains.   I found that you don't want to cover too much area because it might dry on making it more difficult to remove (especially if working on a hot, sunny day).
  • Rinse it Off.  At least, that is what the instructions say.  However, I decided to scrub it in with a mild brush before rinsing it off as an extra measure.  Not sure if that is necessary but figured it couldn't hurt.
  • Wax Your Boat.  This might sound like a pain but it is an important step.  The process of washing your boat and applying the Hull Cleaner will remove any existing wax.  Therefore, it is important to reapply a wax layer to protect the paint or gelcoat.  This should also deter the scum line from forming quickly.  I use the Starbright Premium Marine Polish for this step.  It is actually a "synthetic wax" or polymer and will last about three times longer than a natural (Carnauba) wax.


Here is a before and after picture of my boat.  As you can see, the scum line completely disappeared thanks to the Instant Hull Cleaner.

Scum Line on a Boat

One of the things I love about this product is that it melts away the scum line easily but is not too caustic like many other "more aggressive" hull cleaners.  Its main active ingredients are Oxalic and Glycolic acids which are natural compounds derived from plants.  It is safe to use (unless you ingest it or get it in your eyes).  However, you should still wear rubber gloves when applying it because it may irritate your skin.

However, if you have significant staining or marine growth which is common for saltwater boaters, you might need something more aggressive such as the MaryKate On & Off Hull & Bottom Cleaner.  This has received excellent reviews in previous BoaterInput surveys.  It is significantly more aggressive and contains hydrochloric and phosphoric acids, so you need to be much more careful when using it.  As I have mentioned many times before, you should always use the least aggressive product you can get away with for safety and environmental reasons and to avoid prematurely aging your boat.


Based on my experience with this product, I highly recommend the Starbright Instant Hull Cleaner.  It worked great on my boat and I suspect it would work for yours if you likewise have a moderately dirty hull or scum line.   Not only did it quickly and easily remove the stains with minimal scrubbing, but it was easy to work with and didn't have those toxic fumes of some other products.

Give it a try and let me know what you think.

Jerry Mona - BoaterInput

About the author

Jerry Mona is an avid boater and angler and long-time boating industry insider. With over three decades of experience, he is often considered to be the leading research expert with boaters and has helped numerous manufacturers and trade associations to understand the needs, wants, attitudes and behaviors of boaters. He now shares many of his insights about boats and boaters for free on his BoaterInput.com website.

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