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HOMETUNEDAVE

Two coat epoxy resin

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My boat is 5 years old and I am going to redo the two coat epoxy resin blacking, but I don't know which brand it has on it and the boat builder has gone bust so I can't ask them, will any two coat epoxy resin be OK to apply over the old one or must it be the same one as original. 

Edited by HOMETUNEDAVE

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Broadly, yes. Expect that the existing 2 pack is epoxy modified coal tar aka epoxy pitch.   Most 2 packs were as it was economically advantageous.  Unfortunately coal tar products have now been banned.  Ask your supplier about overcoating epoxy pitch.  AFAIK if it will overcoat epoxy pitch it will overcoat any other epoxy.

 

You will need to pressure wash and then power wire brush or otherwise lightly abrade the existing coating and ensure that any damaged bits are back to bare metal.

Read the new jollop instructions carefully, and ensure the temperatures and humidities required are achieved.  Similarly any curing times needed before refloating.  Otherwise you may well get poor bonding and your expensive new black will come off in big bits.

 

N

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If someone comes to us with a "unknown"' 2 pack system, we use the Jotomastic winter coating which seems to stick to anything else better than guessing. The Dacrylate 2 pack is also good.

All I can say is, if its 2 packed already, try to stay that way, as bitumen seems to be struggling with spilled diesel and galvanic corrosion even on a rigorous 2 year cycle.

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Ask Sherwin Williams in Bolton. They make the stuff and will advise you correctly  sherwin-williams.com    Used to be Leighs Paints before the takeover.

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17 hours ago, HOMETUNEDAVE said:

My boat is 5 years old and I am going to redo the two coat epoxy resin blacking, but I don't know which brand it has on it and the boat builder has gone bust so I can't ask them, will any two coat epoxy resin be OK to apply over the old one or must it be the same one as original. 

 

16 hours ago, BEngo said:

Broadly, yes. Expect that the existing 2 pack is epoxy modified coal tar aka epoxy pitch.   Most 2 packs were as it was economically advantageous.  Unfortunately coal tar products have now been banned.  Ask your supplier about overcoating epoxy pitch.  AFAIK if it will overcoat epoxy pitch it will overcoat any other epoxy.

 

You will need to pressure wash and then power wire brush or otherwise lightly abrade the existing coating and ensure that any damaged bits are back to bare metal.

Read the new jollop instructions carefully, and ensure the temperatures and humidities required are achieved.  Similarly any curing times needed before refloating.  Otherwise you may well get poor bonding and your expensive new black will come off in big bits.

Agree with Bengo. Choose your paint supplier and ask them for recommendations for the specific coating to go over coal tar epoxy. As Bengo says, if it will work over coal tar epoxy then it will work over non coal tar epoxy. Where Bengo says "lightly abrade the existing coating", that means make sure all of the existing coating is abraded. The new coating will not stick to the existing one if not abraded.

 

However, if its only 5 years old, why are you 'redoing' it? Ok there will be patches where the coating will have failed due to mechanical damage which need touching up but these coatings should last 10 years. If you are just belt and bracing it by putting on another coat or couple of coats then great...but if there is a more general failure of the coating with rust breaking through over say 25% of the surface, then you may need to get all of the existing coating off ......and that means grit blasting not water jetting. You say the boat builder has gone bust.......could ask the question "did he paint it properly in the first place"?

 

If the current coating is reasonably sound then you may get away just putting a single coat over everything .....ask the paint supplier for their recommendation....but in this case if the existing coating is a coal tar epoxy, you need to watch out for coal tar bleed through.....so again ask the paint supplier. Your paint supplier will have all the answers so go to one of the better suppliers ie International, Jotun, Sherwin Williams etc.

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1 hour ago, Dr Bob said:

 

Agree with Bengo. Choose your paint supplier and ask them for recommendations for the specific coating to go over coal tar epoxy. As Bengo says, if it will work over coal tar epoxy then it will work over non coal tar epoxy. Where Bengo says "lightly abrade the existing coating", that means make sure all of the existing coating is abraded. The new coating will not stick to the existing one if not abraded.

 

However, if its only 5 years old, why are you 'redoing' it? Ok there will be patches where the coating will have failed due to mechanical damage which need touching up but these coatings should last 10 years. If you are just belt and bracing it by putting on another coat or couple of coats then great...but if there is a more general failure of the coating with rust breaking through over say 25% of the surface, then you may need to get all of the existing coating off ......and that means grit blasting not water jetting. You say the boat builder has gone bust.......could ask the question "did he paint it properly in the first place"?

 

If the current coating is reasonably sound then you may get away just putting a single coat over everything .....ask the paint supplier for their recommendation....but in this case if the existing coating is a coal tar epoxy, you need to watch out for coal tar bleed through.....so again ask the paint supplier. Your paint supplier will have all the answers so go to one of the better suppliers ie International, Jotun, Sherwin Williams etc.

In my experience if you use the boat then its going to get some scrapes and three (just maybe four) years is a good interval for doing an epoxy repair. Boatyards appear to like to recoat the whole boat rather than just patching up. and this does make a lot of sense. I assume extra coats do no harm and likely do a lot of good? Two coats on the whole boat also makes sense otherwise all the scrapes will only have a single coat. Considering the costs of drydocking, washing and surface abrading the cost of the second coat is not a big deal. We now have 7 coats of two different epoxies on much of our boat and its not falling off. I suppose at some stage we will have to shot blast and start afresh...maybe when we don't fit in locks anymore 😀

 

Joton (Jotamastic 87) and International (Intertuf 362?) are popular epoxies, neither are described as "Coaltar" though I don't really understand what a Coaltar epoxy is (can you explain?)

 

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

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1 minute ago, dmr said:

In my experience if you use the boat then its going to get some scrapes and three (just maybe four) years is a good interval for doing an epoxy repair. Boatyards appear to like to recoat the whole boat rather than just patching up. and this does make a lot of sense. I assume extra coats do no harm and likely do a lot of good? Two coats on the whole boat also makes sense otherwise all the scrapes will only have a single coat. Considering the costs of drydocking, washing and surface abrading the cost of the second coat is not a big deal. We now have 7 coats of two different epoxies on much of our boat and its not falling off. I suppose at some stage we will have to shot blast and start afresh...maybe when we don't fit in locks anymore 😀

 

Joton (Jotamastic 87) and International (Intertuf 362?) are popular epoxies, neither are described as "Coaltar" though I don't really understand what a Coaltar epoxy is (can you explain?)

 

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

There isnt any more, Coal Tar products were banned at the start of 2018

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1 hour ago, dmr said:

In my experience if you use the boat then its going to get some scrapes and three (just maybe four) years is a good interval for doing an epoxy repair. Boatyards appear to like to recoat the whole boat rather than just patching up. and this does make a lot of sense. I assume extra coats do no harm and likely do a lot of good? Two coats on the whole boat also makes sense otherwise all the scrapes will only have a single coat. Considering the costs of drydocking, washing and surface abrading the cost of the second coat is not a big deal. We now have 7 coats of two different epoxies on much of our boat and its not falling off. I suppose at some stage we will have to shot blast and start afresh...maybe when we don't fit in locks anymore 😀

 

Dave, when suggesting using one coat I was expecting that any damaged areas would be taken back to bare steel and then two coats of epoxy applied before a final one coat overall.

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1 hour ago, dmr said:

 I don't really understand what a Coaltar epoxy is (can you explain?)

 

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

Dave,

2 pack epoxy coatings contain 3 types of materials.

  1. active components (usually Bisphenol A and epichlorhydrin) that react together to give the tough 3d network that forms the 'impervious' hard coating

  2. Fillers that are added to give the coating some 'body' so when applied it does drip and flow away (ie TiO2, carbonates etc)

  3. Solvents ie Xylene, Toluene, ethyl benzene etc added to reduce the viscosity of the coating for a range of application techniques.

A while back (in the 1960's), some clever bloke found that you could replace part of the active components with coal tar to give better a better coating. Coal tar is a black, smelly liquid that is distilled out of coal and was a cheap material when 'coal was king'. Coal tar is made up primarily from substituted aromatic and naphthenic rings and is quite reactive so can react with the active epoxy components and become part of the 3d network. It's benefits for epoxy coatings are that it improves water resistance (as it is very hydrophobic) and also make the final coating more flexible so not as brittle. Before cure it is also a liquid so less solvent needs to be added to the formulation. Finally it also gives the coating a bit of surface tolerance when applied ….a big feature for its use in things like splashzone coatings for offshore oil rigs.

Unfortunately coal tar contains a lot of chemicals that are carcinogenic so it is rightly banned....but has never been replaced by the boffins. Whilst most paint companies now supply 2 pack epoxies without coal tar, are they as good? I've never been convinced they can get the water resistance, the flexibility and the surface tolerance all as good at the same time. Very similar to Red Lead.

  • Greenie 1

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30 minutes ago, Dr Bob said:

Dave, when suggesting using one coat I was expecting that any damaged areas would be taken back to bare steel and then two coats of epoxy applied before a final one coat overall.

If there are just one or two scrapes in the epoxy this might be the way to go, but if there are lots then its probably safer and not much more effort to do the entire boat, I know from patching up other paintwork that its just too easy to miss one or two of the spots.

 

Is the eventual failure of epoxy down to very slow water penetration? and if so is periodic re-coating a good life extending approach?

 

Next dry-docking in two years time will be our third epoxy repair and I have to decide whether its just another repair or a "shot blast and start again". I would be happy with a repair but I think its time to do the baseplate so if we are getting that blasted then maybe should do the whole boat???????????  Am slightly tempted by the Debdale hot zinc thing.

 

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

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26 minutes ago, Dr Bob said:

Dave,

2 pack epoxy coatings contain 3 types of materials.

  1. active components (usually Bisphenol A and epichlorhydrin) that react together to give the tough 3d network that forms the 'impervious' hard coating

  2. Fillers that are added to give the coating some 'body' so when applied it does drip and flow away (ie TiO2, carbonates etc)

  3. Solvents ie Xylene, Toluene, ethyl benzene etc added to reduce the viscosity of the coating for a range of application techniques.

A while back (in the 1960's), some clever bloke found that you could replace part of the active components with coal tar to give better a better coating. Coal tar is a black, smelly liquid that is distilled out of coal and was a cheap material when 'coal was king'. Coal tar is made up primarily from substituted aromatic and naphthenic rings and is quite reactive so can react with the active epoxy components and become part of the 3d network. It's benefits for epoxy coatings are that it improves water resistance (as it is very hydrophobic) and also make the final coating more flexible so not as brittle. Before cure it is also a liquid so less solvent needs to be added to the formulation. Finally it also gives the coating a bit of surface tolerance when applied ….a big feature for its use in things like splashzone coatings for offshore oil rigs.

Unfortunately coal tar contains a lot of chemicals that are carcinogenic so it is rightly banned....but has never been replaced by the boffins. Whilst most paint companies now supply 2 pack epoxies without coal tar, are they as good? I've never been convinced they can get the water resistance, the flexibility and the surface tolerance all as good at the same time. Very similar to Red Lead.

 

So does (did) the coaltar take a real part in the two pack reaction or was it a sort of chemical filler? As people here say it can react with later coatings I assume its a bit more like a filler and never fully set?

 

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

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20 hours ago, dmr said:

 

So does (did) the coaltar take a real part in the two pack reaction or was it a sort of chemical filler? As people here say it can react with later coatings I assume its a bit more like a filler and never fully set?

 

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

Yes, part of the coal tar will react in with the 2 pack and form part of the 3d crosslinked network. The coal tar is a huge mixture of molecules all containing aromatic or naphthenic rings but with maybe long side cains. Some aromatic/naphthenic rings will react in, some others may not. The ones that do may have long chains dangling. The final coating therefore will have some coal tar not reacted and some parts of the reacted bit 'sticking out'.

When re-coating after a number of years, the solvent in the new coating will extract a bit of the unreacted coal tar so you get bleed through and staining of the new coating.....not really a performance issue, more a cosmetic problem. Will it happen? It will be up to the supplier of the new coating to tell you. The new coating may be formulated with enough filler so you dont see a problem. The chemistry of all of this is very imprecise. Very few chemists (if any) will bother understand what is going on and most manufacturers will just test and see. Coating chemistry is a black art and driven by experience rather than theory. A good coating company ie International, Jotun etc will have tested their coatings over aged coal tar epoxy and will know from accelerated testing (and field trials now) that it is ok. That's why it is important to go to a good company and ask them. A yes it works...is a good answer.  A "well we're not sure" means they havent tested it properly so avoid.

The answer to recoating aged epoxy is to abrade the surface to get a good key. Aged epoxy is a pig to get anything to stick to so everybit of the surface needs to be abraded.

Edited by Dr Bob

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Glad I used Zinga on mine, it seems that the best stuff to use in the 2 pack route has gone, and given when I did mine it would have been coal tar that I used I suppose.

  • Greenie 1

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5 hours ago, Dr Bob said:

Yes, part of the coal tar will react in with the 2 pack and form part of the 3d crosslinked network. The coal tar is a huge mixture of molecules all containing aromatic or naphthenic rings but with maybe long side cains. Some aromatic/naphthenic rings will react in, some others may not. The ones that do may have long chains dangling. The final coating therefore will have some coal tar not reacted and some parts of the reacted bit 'sticking out'.

When re-coating after a number of years, the solvent in the new coating will extract a bit of the unreacted coal tar so you get bleed through and staining of the new coating.....not really a performance issue, more a cosmetic problem. Will it happen? It will be up to the supplier of the new coating to tell you. The new coating may be formulated with enough filler so you dont see a problem. The chemistry of all of this is very imprecise. Very few chemists (if any) will bother understand what is going on and most manufacturers will just test and see. Coating chemistry is a black art and driven by experience rather than theory. A good coating company ie International, Jotun etc will have tested their coatings over aged coal tar epoxy and will know from accelerated testing (and field trials now) that it is ok. That's why it is important to go to a good company and ask them. A yes it works...is a good answer.  A "well we're not sure" means they havent tested it properly so avoid.

The answer to recoating aged epoxy is to abrade the surface to get a good key. Aged epoxy is a pig to get anything to stick to so everybit of the surface needs to be abraded.

Interesting stuff, thanks.

A bit counter-intuitive for me....I always reckon anything will stick to a non-gloss surface and old epoxy is pretty matt.....but then the yard who do ours do a huge amount of surface abrasion so I suspect you are correct.

I now have some experience of three different epoxies and they are very different, I don't know which one is the best though.

 

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

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