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.

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Rust.

Fiberglass.

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.

Rust Stains