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Little black marks

Printed From: Yachts and Yachting Online
Category: General
Forum Name: Repair & maintenance
Forum Discription: Questions & tips on the subject
Printed Date: 30 Oct 20 at 4:41pm
Software Version: Web Wiz Forums 9.665y -

Topic: Little black marks
Posted By: stuarthop
Subject: Little black marks
Date Posted: 23 Feb 05 at 2:25pm
There are a few small black marks under the varnish of my scorp and i would like toget rid off them, a freind told me that you can just sand them away but another told me theycan come back if you do this. does any one know how I can sort this problem?


Posted By: bigwavedave
Date Posted: 23 Feb 05 at 5:51pm

Get a plastic boat, paint it not varnish.

It depends on what the marks are.  If they are damp spots try the following.  I remember my Dad doing it to a Firefly a long time ago.

Get the varnish off and make sure the timber is very dry. If they don't go with a little light sanding then you may be able to bleach them out. But make sure you try on a hidden area first cause you might get a bad reaction. Can't remember what he used at the time but I'm sure a chandlers would be able to put you in the right direction.


Posted By: iansmithofotley
Date Posted: 23 Feb 05 at 8:26pm

Hi Stuart,

You could try masking off, sanding back, drying out, solution of oxalic acid crystals (from chemist shop) and water, dry out and leave, clean up with meths or other spirit to neutralise, dry out, fine sand, epoxy, fine sand, multi varnish job.

If you can get to the under side of the job, it is worth checking that the damp is not working its way to the top side from the under side, if it is, sand back dryout, epoxy and paint/varnish job. It's a good idea to find out what caused the problem in the first place, if the problem is due to flexing, some beefing up may be required.

Ian   (Yorkshire Dales S.C.)

Posted By: bigwavedave
Date Posted: 23 Feb 05 at 9:38pm
Thats the stuff oxalic acid, and I think he then used white spirit or maybe turps.


Posted By: stuarthop
Date Posted: 24 Feb 05 at 9:53pm

k thanks for the help guys i'll try all that 2moro



Posted By: KnightMare
Date Posted: 28 Mar 05 at 7:06pm

Yeah I have a similar problem on my sisters boat but the boat is epoxyed instead of varnished can i this just be sanded back in the same way or not?

All of these black bits have been caused by chips in the epoxy. I just want to remove the colour before covering them up, do i just do the same as was advised for stuart.


Posted By: Lucy Lee
Date Posted: 29 Mar 05 at 10:05am
Yes, epoxy can be sanded, but it will be harder than sanding varnish (start with reasonable coarse grit sandpaper). I don't know it there are potential interactions between epoxy & turps etc so it would be worth testing that out first.

Fly Cherub!

Posted By: StephenSails
Date Posted: 29 Mar 05 at 9:49pm

Originally posted by bigwavedave

Thats the stuff oxalic acid, and I think he then used white spirit or maybe turps.


Its good stuff, mix it warm water and be careful as its quite bad for you!




Posted By: Buzz
Date Posted: 30 Mar 05 at 10:54am

Oxalic acid is a well known bleaching agent. I have used it on Firefly's and it works well.

Be careful it is poisonous if swallowed and is a severe irritant if inhaled or absorbed through the skin.

The link below gives a detailed description and precautions. -

Posted By: stuarthop
Date Posted: 30 Mar 05 at 4:27pm
my scorps epoxy coated and it worked fine thanks guys.


Posted By: KnightMare
Date Posted: 02 Apr 05 at 9:18pm

yeah Buzz it has now been classed as a top class poison (or some thing like that) so i had to get my dad to go and collect it from the very nice chemists shop that ordered it in for me.

My next problem is that since i have a pot of this stuff what concentration should i mix? Any advice on this bit?


Posted By: neilw
Date Posted: 02 Apr 05 at 11:35pm


You should be OK with 4:1, best to trial on a non-obvious place

There are also commercial products that contain oxalic acid, but are "safe" to use.

I have used Net-rol in the past and there's info on another product,TBS, over on the Classic and Vintage Racing Dinghy Association's forum. - This link should take you to the relevant thread


Posted By: nixy30633
Date Posted: 03 Apr 05 at 12:42pm
hi, i need to clean up the black marks on my boat. i have the oxalic acid crystals, but when you put it on the boat do you just leave until it has dried out and then neutralise with meths or another spirit.

Posted By: iansmithofotley
Date Posted: 03 Apr 05 at 8:48pm

Hi nixy30633,

Personally, I would brush on the solution of crystals and water and let it dry out.  If after a few days, the black marks are still there, then I would repeat the procedure, as many times as necessary, until you are satisfied with the result. Once you are happy with the job, then you should use the neutralising spirit and leave it to dry out again.  I suppose that there will be some marks which will never come out no matter how many times you repeat the procedure, it just depends how bad they are. I am not an expert on this subject but I once refurbished a wooden Enterprise and the method worked for me.

Ian  (Yorkshire Dales S.C.)

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