Remove antifoul but leave 2-pack epoxy?

Ladybirdgaffcutter

Ladybirdgaffcutter

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How would you go about this please, it's outside my experience?
Hull material is 100 year old rivetted iron. It's got plenty of old, inactive corrosion pitting.
It's a flat bottomed tjalk (sailing barge), will be on blocks in dry dock.
The old pitch coating was previously media blasted off, and 2 pack epoxy paint applied, then the whole thing was antifouled. Done a few years ago now, so I want to check it thoroughly.
How could the antifouling be removed without hurting the expensive 2-pack under it? Or perhaps it can't be done like that?
Maybe media blasting, but which media and technique? I will naturally ask the shipyard, but real world experiences and 'don'ts' are very useful as well..TIA
 
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Ladybirdgaffcutter said:
How could the antifouling be removed without hurting the expensive 2-pack under it?
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I found that Removall paint stripper removed antifouling and one-pack, but had no effect at all on two-pack. However, the stripper for a 5m sailing boat cost about £30, so it might be prohibitive for a tjalk.

Recently I had layers of old paint removed from a GRP dinghy by soda blasting. That left the gel coat - which has similar properties to two-pack - matte, but otherwise unaffected. Again, not a cheap option.
 
Having spent half a winter scraping AF from a 24 footer, I'd go for the blasting, in the hands of a good operator, it'll do less harm to the epoxy than any other method. There's a variety of materials, including dry ice, but I've no idea which would be the most appropriate for your job
 
Ladybirdgaffcutter said:
How would you go about this please, it's outside my experience?
Hull material is 100 year old rivetted iron. It's got plenty of old, inactive corrosion pitting.
It's a flat bottomed tjalk (sailing barge), will be on blocks in dry dock.
The old pitch coating was previously media blasted off, and 2 pack epoxy paint applied, then the whole thing was antifouled. Done a few years ago now, so I want to check it thoroughly.
How could the antifouling be removed without hurting the expensive 2-pack under it? Or perhaps it can't be done like that?
Maybe media blasting, but which media and technique? I will naturally ask the shipyard, but real world experiences and 'don'ts' are very useful as well..TIA
Click to expand...

A good operator of a slurry blast can do it. But with flat bottomed only if the blocks are big enough as you need space.

Solvented epoxies have a dry film thickness of 45-50 microns and are applied in alternating colours. Green grey for international. After blasting by a good operator you will be left with a green grey mottle, this proves that an average of 45 microns of epoxy has been eroded.
It would be similar on a solvent free epoxy which is thicker, but you wouldn’t have the colour guide.
So after blasting I would apply a coat or even 2 of solvented epoxy (which is easy to apply) and restore the thickness of the underlying epoxy. It will go straight on your blasted “2 pack” with no further prep.
Chemical stripping is messy, you end up damaging your epoxy with scrapers and you need loads more than you think so it isn’t cheap.
Find a slurry blaster used to working on GRP ie is skilled and gentle. Steel blasting blokes will strip your epoxy.
 
You may find, as I did, that using 'soda' as your blast material is very much gentler - in the hands of a reasonably competent operator - than more aggressive stuff. I was persuaded by a very experienced paint specialist to try this, so I hired the kit and did it myself, slowly and cautiously.... and cost effectively. The used media dissolves in water, but the removed a/f is toxic, of course, to wildlife and watercourses, so gathering the debris up on tarps and proper disposal is indicated.
 
Don't use any outfit that offers grit blasting. They won't just remove the 2 part epoxy they'll do their best to take the gel-coat off as well.
 
Daedelus said:
Don't use any outfit that offers grit blasting. They won't just remove the 2 part epoxy they'll do their best to take the gel-coat off as well.
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Its a rivetted iron boat-so ergo, no gel-coat.

If there is any rust or corrosion under the antifoul, I would expect careful examination to find areas showing this. There will be rust showing, or antifoul and epoxy coating lifting and showing bulges or bumps.

If none are found, just apply antifoul as normal.

I have a similar job to do soon. Our Kiwi boat, a 1986 Hartley 32 in corten steel was last antifouled three years ago. It should be lifted and pressure washed at the weekend or early next week.

If no rusty patches or bulges/bumps are apparent under the antifoul, fresh will be applied. If any such faults are found, they will be dealt with locally as required.
 
If you do find corrosion & you probably expect to find some - that is, presumably, why you are checking- you are going to have to remove areas of epoxy to get to the areas to be re treated. That could be extensive, or not, as the case may be. In any event it may be a re visit for the grit blasting team &, presumably, further expense.

So why not bite the bullet & just have the whole thing grit blasted off in one go. You will not have areas of damaged epoxy ( by whatever method you have used) which will need re treatment, or which will fail, if missed,in a couple of years, needing further early repeat of the operation.

By ripping it all off by grit blasting it will be cheaper. Any corrosion will be removed. You will know how effective the epoxy was, thus giving guidance of when to repeat the operation next time.

Then re epoxy. Re antifoul. It will be quicker. You will have a good idea of time scale for the next time. Sometimes a complete re epoxy can be as cheap as a repair job especially as the antifoul removal may damage some of the existing underlaying epoxy without you knowing.
 
Daedelus said:
Don't use any outfit that offers grit blasting. They won't just remove the 2 part epoxy they'll do their best to take the gel-coat off as well.
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Have you not read the OP's post. It is an iron boat.
But as for grit blasting a GRP boat. Mine was grit blasted & the chap did an excellent job. I would suggest that your comment may apply to unskilled operatives with no understanding of the task in hand. However, that can be applied to many tasks . Not just antifoul removal.
 
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