Monday, June 3, 2019

Rust Stains 

Over and over again I see people querying for a method to remove rust stains from fiberglass.  And over and over again I have typed in a quick answer.  Because my quick answers are not always complete, and also because I am getting frustrated at answering the same question over and over, I am writing this post so that I can just refer to it.



The surface of fiberglass (gelcoat, actually) is porous at the molecular level, that is why it stains so easily (spill a glass of wine and you'll see what I mean...).  And so it is with rust stains - they are not *on* the surface, but *in* the surface.

So what is the best way to remove rust stains from gelcoat?  Certainly one method is to simply remove the rust contaminated gelcoat - that is what abrasives do.  Besides the elbow grease required, this approach is limiting because eventually you will run out of gelcoat.

And then there is chemical treatment.  Boy have I seen a wide range of suggestions here:
  • Distilled white vinegar
  • Cider vinegar
  • Coca Cola
  • Pepsi Cola
  • Ospho
  • Whink
  • Bar Keeper's Friend
  • Starbrite
  • Scotchbrite
  • FSR
  • Clay Bar
  • Rubbing compound
  • Bleach (numerous variants here...)
  • ...
Some of these are abrasives, and as mentioned above, they work by removing the stained gelcoat.

Some of the non-abrasives will be marginally effective.

But for a sure-fire, elbow-grease free and effective solution, use oxalic acid.

Oh no!  Acid sounds scary!  Must run away!  Oxalic acid is a weak acid, in the same vein as vinegar but a little stronger.  Do you wear rubber gloves when handling vinegar?  I didn't think so.

Oxalic acid is a crystalline solid - looks a lot like sugar.  To be effective, it must be used in solution.  Now here is an interesting fact:  oxalic acid is way, way more soluble in hot water than cold...  so when making a solution, always use hot water.  And always make a saturated solution (that is, no more will dissolve in the water - you can tell because there are still a few undissolved crystals on the bottom).

So just make up a saturated solution of oxalic acid, wet a piece of paper towel with the solution and stick it on the stain.  That is all.

Also some notes:
  • Bar Keeper's Friend is a soft abrasive with a small amount of added oxalic acid.
  • The active ingredient in FSR is oxalic acid.
  • Whink contains HF - hydrofluoric acid.  HF is scary stuff - it will even dissolve glass.
  • Bleach will be completely ineffective.  In fact it may serve to set the stain.
  • Phosphoric acid (Ospho, Coca Cola, Pepsi Cola) will be marginally effective
 So how would your average boat owner be able to obtain this magic oxalic acid?  Actually it is quite easy.  Oxalic acid is sold at Home Depot, etc. as "wood bleach"... look for it in the paint section.  Now why would this be?  Because the dark brown color in wood is due to iron oxide - rust.  And by the way, the dark brown staining from tannin-loaded waters is also due to iron oxide - oxalic acid will work on it too.

Now a final note on toxicity.  You have already eaten oxalic acid.  The sour taste in rhubarb is oxalic acid.  But you don't eat the leaves of rhubarb - why?  Because the oxalic acid concentration in the leaves is higher.  As with almost everything, the dose is the poison (even water and oxygen...  drink too much water and it will kill you...  deep sea divers use exotic gas mixtures containing far less than the 21% oxygen in the air because the pressure makes oxygen that much more dangerous).  So don't eat or drink the acid solution.  Don't breathe any dust.  Don't rub it into a cut or use it as an eyewash.  Wash your hands after contact.  It is about 3 times more toxic as a poison than aspirin, and about 1/4 the toxicity of caffeine.  Yes, it is poisonous but no heroic precautions are necessary.



Anonymous said...

I try to keep the diversity of solvents and cleaners on the boat to a bare minimum. Since I already had Barkeeper's Friend aboard for scrubbing in the kitchen, I tried it as a rust remover. I made a thin slurry by adding water and used a toothbrush to apply it )without scrubbing) to the rust stains on my old gelcoat. Even the darkest stains came right out after soaking for about 15 minutes.

Anonymous said...

Great! A friend of mine was swearing by oxalic acid in helping spruce up a boat he was selling. Don Casey claims in "This Old Boat" that it dulls paint or gelcoat - any comments on that? And, any ideas in getting chemistry to help clean soot marks off a painted surface would be hugely appreciated.

Robert Salnick said...

Although our fiberglass is already kind of dull, I can’t say that the oxalic has any more effect on it than vinegar. I have no experience with oxalic acid and paint.

As far as soot is concerned, there is nothing that I am aware of that will remove carbon. However soot is frequently mixed with oily materials, and these are easily wiped up with paint thinner.

Drew Frye said...

I don't know about there, but in much of the country, wood bleach has been removed from the shelves due to toxicity. You won't find it in the local Home Depot, in my experience, but you can order it.

An effective alternative is CLR (~ 24% lactic acid and other organic acids). Although slightly less effective for gelcoat iron stains, it is actually the most effective per unit corrosion for descaling aluminum (did a bunch of testing). It is also considerably safer to work with, though oxalic acid just requires common sense.

Robert Salnick said...

Thanks Drew. I haven’t checked for quite a while because I bought 50 lb of oxalic acid when we lived in a place where the water supply was heavily loaded with iron. We used it in the washing machine to keep our clothes from turning orange and in our dishwasher to keep the dishes from getting iron stained. Still have a few pounds left.

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