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I was going to key my epoxied stern deck today but I won't have time to paint the undercoats and topcoats until next weekend. The epoxy has been on there for many months so it needs a key before the undercoats go on, but how long will the key be good for with sun, rain, etc, before it needs keying again?

 

It's a big deck and I have to take the boat up the river to use a palm sander to placate a (land) neighbour who's been complaining about me using power tools. I could get a coat of undercoat on today but it's raining tonight and also the undercoat has a max overcoating time of 3 days before it needs keying itself. So I don't want to start painting until I can finish all 4 coats, but it would be good to get the deck keyed today so that when I get a few good days free I can start painting.

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We have just been in the dry dock for a second "epoxy repair". Last time the whole boat was sanded with a course sanding pad in a big angle grinder to give the new coat of epoxy a key to the old epoxy. This time during the pressure wash I noted that quite a bit of the new coat came off. The conclusion here is that no matter how good your prep/keying is, it is much better to apply epoxy coatings before the previous coat has set to get a proper chemical bond.

 

Also, I did our decks in epoxy and we use gaffer tape on the back deck to seal the weedhatch cover. Removing this did lift a bit of the epoxy revealing very shiny metal below. Epoxy does not stick that well to a smooth surface so trying to get a bit of texture is important......shot blasting and needle gunning is better than sanding.

 

.................Dave

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How are you cleaning the dust off the keyed surface before applying subsequent coats? I always dust and then spirit wipe with the appropriate thinners and never have a problem with the epoxy bonding to steel or to itself.

 

In this case the epoxy is well bonded to the steel and to previous coats. I've been using Jotamastic for many years so I do follow the overcoating times and prep guidelines and I've used the combination of keyed epoxy/undercoat many times when I haven't been able to get the undercoat on within the max overcoating time. However usually I just key and then paint straight away, but this weekend that isn't possible so I was wondering how long the key will last if I do it today?

Edited by blackrose
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The key is effectively mechanical damage to the previous coat whch provides either for mechanical attachment of the subsequent coat or to allow chemical access to the part cured paint below the surface.  In your case the existing coat seems fully cured so the key is purely for a mechanical bond.  It should therefore last indefinitely,  or until you wear the keyed coat too smooth for mechanical attachment. 

 

N

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1 hour ago, BEngo said:

The key is effectively mechanical damage to the previous coat whch provides either for mechanical attachment of the subsequent coat or to allow chemical access to the part cured paint below the surface.  In your case the existing coat seems fully cured so the key is purely for a mechanical bond.  It should therefore last indefinitely,  or until you wear the keyed coat too smooth for mechanical attachment. 

 

N

 

Ok. I understand what you're saying, but I thought there was a chemical element to a keyed surface too, because by keying cured paint you're also opening up the molecular structure of the surface to which the new coat forms molecular bonds?

 

I have to admit my knowledge is sketchy on the point at which a purely mechanical bond becomes a molecular/chemical bond, or even if a purely mechanical bond actually exists? I think all mechanical bonds may be molecular to some degree.

 

We probably need Dr Bob to comment.

 

Anyway the weather looks shite today, so if I do the prep this weekend I won't be doing it until tomorrow.

Edited by blackrose
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Interested in this topic as I have just spent £2500 on two-pack paints and I want to try and get it 'right'

 

Assuming you are talking about your primer / undercoat have you checked the data-sheets.

 

My Hempel data-sheet shows a maximum overcoating time of :

 

30 days at 20*C, or 60 days at 10*C

 

"if the maximum overcoating period has been exceeded, clean with fresh water and allow to dry. Lightly abrade and remove all dust prior to repainting"

 

It doesn't give any guidance on how quickly you must overpaint before having to 'abrade' again.

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Check those maximum overcoating times Alan because they're usually different depending on whether you're overcoating to "itself" with more coats of the same epoxy, or overcoating your final epoxy coat with an undercoat.

 

Edit: I use a single pack Hemple Primer/Undercoat on top of the Jotamastic epoxy. If you're using a two pack Hemple undercoat then it might be the same max overcoating time.

 

I haven't seen any guidance on how long a key lasts in any of the data sheets either which is why I posted the question. I think in general it's best to get the next coat on as soon as you can after keying, but I don't know if say a week is too long, especially if the keyed surface gets wet and oxidises for example.

Edited by blackrose
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27 minutes ago, blackrose said:

Check those maximum overcoating times Alan because they're usually different depending on whether you're overcoating to "itself" with more coats of the same epoxy, or overcoating your final epoxy coat with an undercoat.

 

Edit: I use a single pack Hemple Primer/Undercoat on top of the Jotamastic epoxy. If you're using a two pack Hemple undercoat then it might be the same max overcoating time.

 

I haven't seen any guidance on how long a key lasts in any of the data sheets either which is why I posted the question. I think in general it's best to get the next coat on as soon as you can after keying, but I don't know if say a week is too long, especially if the keyed surface gets wet and oxidises for example.

Thanks - I do want to get it right - I'm not wanting to waste 100's of hours and £1000's on a bad job.

 

Everything is two-pack.

 

My 1st coat is going to be 2-3 coats of "High Build High Protect Epoxy Primer"  which 'fills and smooths' the surface.

 

Then will be the 'Primer / Undercoat 45551'

The Data sheet for the 2-pack Primer / undercoat states that the overcoat times are based on the 'subsequent' coats being :

Hempel Ocean Gloss,  Hempels Polyenamel or Hempels Polygloss.

 

I'm using the Polygloss  55530 and for the non-slip walkways I'm using "Hempel Non-Slip deck Coating 56251"

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A keyed surface can become contaminated, and embedded with contaminants. I'd refresh the mechanical keying up and thoroughly clean and degrease. I probably wouldn't rely on water to do the cleaning job. Vacuum, tack rag, surface cleaner (degreasing). You're going to be relying on the mechanical bonding on fully cured paint.

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