Yellow-brown hull staining

[kieronriley]

You can use any lime scale removing toilet cleaner .its already jelly like. (So needs no thickening).its already mixed (so dosnt need to dissolve) it is usually coloured (so you can se where you have applied it) just apply with a dry cloth then stand back and watch the crap disappear ,then rinse off ,always of course using Marigolds , and take care not to splash in you're eyes , Oh and you can usually buy it for about one pound (so it's cheaper than anything else) Kieron

 

[Yngmar]

My powdered oxalic acid powder dissolved very easily.

It does indeed, especially in warm water. What may have happened is that they sprinkled in too many crystals - once saturated, it will no longer dissolve at all. In fact that's a good way to tell you've put in enough.

 

[GAJ52]

I have just read this thread and about to post a similar question.

My boat is about 20 years old with white GRP, and I understand it was positioned on hard standing for several years before I bought it. Where the water collected on the walkways the Gel Coat has become stained with a yellow colour that will not wash off even with an abrasive cleaner.

Is this the same 'yellow' problem mentioned by the OP and will the product Yachticon Anti-Yellow restore the colour ?

 

[Hydrozoan]

I know how to get it off (Yachticon Anti-Yellow works a treat) but is there any way to stop it getting on there in the first place?

I don't think there's much that can be done by way of prevention, but there's some discussion here of possible causes which may be of interest:
http://www.ybw.com/forums/showthread.php?427092-Brown-water-line-stains&highlight=brown+waterline

 

[johnalison]

I have just read this thread and about to post a similar question.

My boat is about 20 years old with white GRP, and I understand it was positioned on hard standing for several years before I bought it. Where the water collected on the walkways the Gel Coat has become stained with a yellow colour that will not wash off even with an abrasive cleaner.

Is this the same 'yellow' problem mentioned by the OP and will the product Yachticon Anti-Yellow restore the colour ?

I'm not sure what you mean by walkways, maybe around the deck or cockpit, but I rather suspect this may be quite different, but there's no harm trying an oxalic-based product such as Yachticon or Y10.

 

[GAJ52]

Sorry not sure of the correct term, but the 1 foot wide walkway around the perimeter of the boat - Birchwood 37

 

[William_H]

Sorry not sure of the correct term, but the 1 foot wide walkway around the perimeter of the boat - Birchwood 37

I would try oxalic acid (or even lemon juice) first. If this does not clean it in 5 minutes waiting then try bleach. If that does not clean it try caustic soda. If all else fails try acetone. Just hose off the previous cleaner before trying the next.
good luck olewill