Oxalic acid not removing all waterline stains. recommendations please

My recipe for stain remover: Dissolve as much crystalline oxalic acid in 500ml hot water (tap hot, not boiling) as will dissolve, then squirt in about 4-5 tablespoons of household washing up liquid (Fairy or similar).
Then scrub on with a brush. Rub enough to get a lather, this slows evaporation, and also helps remove things that detergent will remove as well as the oxalic doing its job.
 
Once you have mixed up your Oxalic Acid solution then bung a packet of wallpaper paste in the mixture. Stir until you have a "gloopy" consistency and then trowel on with a large brush. Leave for 15 mins and then either hose or pressure wash off.
Trouble with using the plain solution without wallpaper paste is that it runs off the surface you want to clean.
 
lydiamight said:
Once you have mixed up your Oxalic Acid solution then bung a packet of wallpaper paste in the mixture....
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I'd really avoid the wall paper glue solution. Go for what MM5AHO suggests above. Fairy Liquid. Make it very "concentrated (e.g. 1 vol FL : 1 vol saturated oxalic acid). Then the viscosity of the household detergent will keep the aqueous film on the vertical side long enough for it to act (wait for a day that is warmer than today!!!!). The advantage, as he says, is that the detergent also helps remove the oily deposits. But the most important thing is that it is very easy to wash off (unlike the wall paper glue).

Oh yes, and I also would recommend the CIllit Bang (orange) as an ADDED treatment, but separate from the oxalic acid on. With LIGHT abrasion with a stainless steel wool scrub (the sort that can be purchased for normal kitchen use)
 
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Just one thought: if it's a mess due to hydrocarbon residue, rather than some inorganic stain, the best thing I have found is acetone.
That's the only thing I have found to have an effect on fender-gunge and Round-the-Isalnd stickers.
Of course, I realise that wouldn't really be a "stain", then, but perhaps worth a try to see if it has an effect.
 
I have only used oxalic acid in its Y10 form and I can definitely confirm that it only works on a warm day!

The technique I use is:

1. Paint one side of the hull with a thin coat of Y10, using as wide a brush as will fit in the pot.
2. Go back to the end I started from and wash off the Y10 with hot soapy water and a scouring pad using a circular motion
http://janitorialsupplies.co.uk/scouring-pads-green.html?gclid=CLPc4Iqc-soCFQoUwwodr6QHNw .
3. Rinse off with fresh water from the hose.
4. Repeat for the other side and finally the transom and rudder stock.

My grp hull is 49 years old and still comes up clean and white every year with this treatment.
 
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