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Old 14-09-2010, 04:02   #1
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Rewhitening Yellowed Gelcoat

Ive been reading about Oxalic acid & wallpaper paste to whiten uv-yellowed gelcoat, but despite the apparent enthusiasm for this method there is not much detail on how strong the solution should be, how long to leave it on, etc.

So, I'm hoping some of the kind folk in this forum can provide some tips from people who have used this method before - or at least a pointer to a good set of instructions.

Ive got a 2.5kg tub of Oxalic acid crystals (99.6%), and the fixings for some good old fasioned flour & sugar paste. Does anyone know please:

1. How many grams (ounces) oxalic acid crystals should be diluted per litre (gallon) of water for deyellowing uv-yellowed fibreglass?

2. How long should I leave it on before washing off?

3. Will it damage aluminium toerails/portlight surrounds? Other deck hardware?

4. Will it damage/polish polycarbonate or perspex windows?

5. Will it deglaze shiny fibreglass?

6. Do I have to polish or wax immediately to protect or can it wait a few days?

Any other sage words on this topic?

Thanx in advance!
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Old 14-09-2010, 05:42   #2
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Is the paste just a vehicle for keeping it on vertical surfaces? Oxalic acid will remove a lot of stains, but if your gelcoat has truly turned to a yellow color and not just yellowed wax, imbedded dirt or other stain, I doubt oxalic acid will whiten it.

I just mix oxalic acid in water in a spray bottle and spray it on. I use warmish water and stir in the OA until it doesn't dissolve anymore. Spray it on, wait until the color disappears and rinse it off. If it dries or takes more to get rid of the color - reapply.

It will damage aluminum and tarnish bronze (actually clean it, but you will see it as discolored). No problem with stainless (you can use it to remove rust stains). I don't know about polycarb or perspex, but I wouldn't get it on them either. OA has some ability over time to make gelcoat porous if the gelcoat is going bad anyway. This will result in a dull look, but the gelcoat can be rebuffed and waxed after. It is always best to protect clean gelcoat as soon as possible, but it can wait a few days.

Best to wear gloves when applying and a particle mask when mixing and applying as a spray because OA isn't very friendly to humans. Inhalation shouldn't be a problem if you apply as a gel.

If you just have a stained waterline, "On And Off" is a product containing hydrochloric acid. It works like magic on waterline stains. It will also work on the whole boat, but is very detrimental to aluminum and other parts, which makes it difficult to apply well.

Mark
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Old 14-09-2010, 16:23   #3
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I use toilet bowl cleaner, cheap and works fine
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Old 14-09-2010, 17:02   #4
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I use toilet bowl cleaner, cheap and works fine
So do I.

I use "Snow-Bol". It's cheap, it works and you find it at Ace Hardware. It's Lye based ... and removes rust as well.

Remember to wear gloves.
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Old 14-09-2010, 17:20   #5
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Sno-bowl plus comet cleanser is a classic. But as Tropic Cat states, wear gloves, etc. The combo works well but is REALLY strong. Let it drip off the sides and you will most likely remove any bottom paint above the waterline. The only time I use the combo is just before hauling the boat for a bottom job.

I do know of some others that use pool shock only slightly diluted but can't give any personal experience. I would suspect that the gelcoat may just be getting old and porous and you will have a paint job in your future.

I feel your pain , I can't afford a full paint job but I have and can, at least, repaint the non-skid without too much heartburn. That's on the list for this fall or winter.

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Old 14-09-2010, 17:27   #6
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do you guys just wash that off into the water?
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Old 14-09-2010, 18:07   #7
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do you guys just wash that off into the water?
Yes

I apply small quantities of the cleaner in conjunction with a wire brush. I then pour a ton of fresh water on the patch I've completed to thoroughly dilute it before it hits the Lagoon.

It's not a perfect system but my manatees don't seem to mind!
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Old 14-09-2010, 18:13   #8
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I use a product called whink< contains oxylic acid. simply brush on just enough to wet then wash after color changes.
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Old 14-09-2010, 18:14   #9
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Quote:
Originally Posted by Tropic Cat View Post
Yes

I apply small quantities of the cleaner in conjunction with a wire brush.
You use a wire brush on your gelcoat? Besides the abrasiveness, I assume it is a stainless brush and not carbon steel.

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Old 14-09-2010, 19:17   #10
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I have to. My entire deck is molded non skid surface. The brush gets into the heavy texture.
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Old 14-09-2010, 20:14   #11
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That's understandable, non-skid is always tough to clean - but wire? Won't a stiff scrub brush work? It seems like wire could gouge and scratch and leave bits to rust. I use wire brushes to clean steel...

Or maybe I'm misunderstanding what you mean by wire brush - I picture the ones you descale metal with.

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Old 15-09-2010, 04:56   #12
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Thanks everyone!

As I've already got the oxalic acid, I'll give that a shot first. If that fails I'll try the throne cleaner. I'm in Australia and we don't have sno bowl or comet, but we do have to clean our toilets on the odd occasion so I'm sure I'll find something to match, chemically speaking .

I still haven't found a suggested ratio of water/oxalic crystals so Ill start off experimenting with a 10:1 ratio of water to crystals. I want it in a paste so it doesn't run all over the place and dribble onto areas I don't want contacted (like the aluminium toerails - thanks colemj!) and so I can have it sit on vertical surfaces in the cockpit.

I'm going to try this over the next few days so I'll let you know how it goes. I'll try to remember some before/after pics as well.
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Old 15-09-2010, 05:17   #13
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In the states I use a product called On & Off Gel which is simply and acid based gel cleaner..



Brush on with a cheap chip brush (no scrubbing at all)


Rinse off..


Oxalic crystals will work too but won't be a thick gel. Am curious to hear how the wall paper paste works..
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Old 15-09-2010, 05:31   #14
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Ah yes, Mary Kate's "On and Off". Made a job a living hell for me when it dumped bottom paint polluted water all over the tarmac. Good stuff though. Be careful what ya use it on.

You could always just spray on new Gelcoat....
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Old 15-09-2010, 05:37   #15
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Ah yes, Mary Kate's "On and Off". Made a job a living hell for me when it dumped bottom paint polluted water all over the tarmac. Good stuff though. Be careful what ya use it on.

You could always just spray on new Gelcoat....
You DO NOT want an acid based cleaner to drip on any bottom paint as it will eat the copper out of it and ruin it, especially if it is a multi-season ablative. I make a 14" tall drip skirt out of heavy duty sheet plastic that gets taped to the waterline and hangs free at the bottom edge. Any acid run off rolls down the drip skirt and drips off the bottom edge, not along the hull.
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