To blast or not to blast

[captain flint]

I've read a few comments in various threads that refer to 2 pack being applied without shot blasting. Pretty sure some have even suggested it can be done without going back to bare metal. 

 

Two good outfits I know who do it swear that's wrong, and that it really needs shot blasting to adhere properly. They both talk disapprovingly of those who use it without that level of prep, and how often they have seen and had to deal with problems resulting.

 

You might think, "Well they would say that," - after all they both offer shot blasting and 2 pack application! But I've seen examples of it flaking off when the person who did it swears they got right back to bare metal. And I'm not just talking below the waterline (although I do also know someone who bought a brand new boat by a pretty well known maker whose boats go by his name, where the 2 pack epoxy below the waterline started flaking off within a year). 

 

I don't have any 2 pack plans right now, I'm only stirring, sorry, I mean asking, as I think it will be interesting to hear what people's views are and how they back them up, and what their experiences have been. 

 

 

[Paul C]

https://www.smlmarinepaints.co.uk/uploads/files/library/files/DATA_SHEETS/Jotamastic_90_AG.pdf

[john.k]

There are other factors ,such as has the hull been shotblasted before ........if it has already been blasted ,then it can be done with HP water ...thats at 30,000psi.+

[Tonka]

https://www.smlmarinepaints.co.uk/products/Epoxykit65

 

Can be used without shotblasting

[jonesthenuke]

When you have chosen the two pack paint system that you are going to use, download and read the instructions carefully.  It is highly likely (near certain) that it will specify that the substrate should be shot blasted, usually to a standard of Sa2.5. If you choose to do any less than the paint manufacturer specifies then you are likely to be disappointed by the outcome.

 

Note the same applies to meeting all the requirements, such as ambient temperature, humidity, curing time before re-floating the boat etc.

 

Any boat yard saying they know better and that you do not need to comply with the manufacturer's specification is at best taking a chance and would make me doubt their competence to do the work. 

 

After all its worth doing the job properly as it should last many years if done correctly.

[Tracy D'arth]

Every epoxy 2 pack I have used advises grit blasting to the appropriate standard.

I have done a hull by scraping with a big nasty Sandvik scraper back to bare steel and then wire brushing but it was hard work and better to grit blast. It stayed on as far as I know, 23 years ago.

[blackrose]

1 hour ago, captain flint said:

I've read a few comments in various threads that refer to 2 pack being applied without shot blasting. Pretty sure some have even suggested it can be done without going back to bare metal. 

 

Two good outfits I know who do it swear that's wrong, and that it really needs shot blasting to adhere properly. They both talk disapprovingly of those who use it without that level of prep, and how often they have seen and had to deal with problems resulting.

 

You might think, "Well they would say that," - after all they both offer shot blasting and 2 pack application! But I've seen examples of it flaking off when the person who did it swears they got right back to bare metal. And I'm not just talking below the waterline (although I do also know someone who bought a brand new boat by a pretty well known maker whose boats go by his name, where the 2 pack epoxy below the waterline started flaking off within a year). 

 

I don't have any 2 pack plans right now, I'm only stirring, sorry, I mean asking, as I think it will be interesting to hear what people's views are and how they back them up, and what their experiences have been. 

 

Any mention I've made on other threads about applying epoxy over existing paint without blasting is either referring to epoxying over existing epoxy (first keying the old epoxy, etc), or using epoxy topcoats onto correctly prepped single part paint on cabin sides, etc, above the waterline.

 

If the below waterline areas of previously bitumened hull are to be epoxied then the correct prep must be done and generally that means some form of blasting to get the bitumen or other single part blacking off - although allegedly there are some new epoxies that can go on top of bitumen. I remain dubious. 

 

 

8 minutes ago, Tracy D'arth said:

I have done a hull by scraping with a big nasty Sandvik scraper back to bare steel and then wire brushing but it was hard work and better to grit blast. It stayed on as far as I know, 23 years ago.

 

Yes, prep with power tools and scrapers is possible, but as you say, for large areas that's very labour intensive.

Edited March 4 by blackrose

[Captain Pegg]

1 hour ago, captain flint said:

I've read a few comments in various threads that refer to 2 pack being applied without shot blasting. Pretty sure some have even suggested it can be done without going back to bare metal. 

 

Two good outfits I know who do it swear that's wrong, and that it really needs shot blasting to adhere properly. They both talk disapprovingly of those who use it without that level of prep, and how often they have seen and had to deal with problems resulting.

 

You might think, "Well they would say that," - after all they both offer shot blasting and 2 pack application! But I've seen examples of it flaking off when the person who did it swears they got right back to bare metal. And I'm not just talking below the waterline (although I do also know someone who bought a brand new boat by a pretty well known maker whose boats go by his name, where the 2 pack epoxy below the waterline started flaking off within a year). 

 

I don't have any 2 pack plans right now, I'm only stirring, sorry, I mean asking, as I think it will be interesting to hear what people's views are and how they back them up, and what their experiences have been. 

 

 

In respect of the new build you refer to it’s probable that the paint remained properly adhered but to mill scale rather than the base steel and it’s the mill scale that has fallen off taking the paint with it.

[Arthur Marshall]

As I've said elsewhere, after overplating, mine was bitumined even though I'd requested epoxy. Next time it came out it was done with epoxy and I am pretty sure it wasn't blasted right back to the metal first (I certainly didn't pay for it) but it stayed on happily for six years, and it's now been epoxied again with no problems. That's anecdotal evidence, I wouldn't want anyone to rely on it, but there it is.

3 minutes ago, Captain Pegg said:

In respect of the new build you refer to it’s probable that the paint remained properly adhered but to mill scale rather than the base steel and it’s the mill scale that has fallen off taking the paint with it.

I did wonder if that had happened to mine - the bitumen being on mill scale and it mostly got knocked off with the power jet when epoxing.

[blackrose]

17 minutes ago, jonesthenuke said:

Note the same applies to meeting all the requirements, such as ambient temperature, humidity, curing time before re-floating the boat etc.

 

Exactly. The correct level of prep is not the only requirement that must be met when using more "technical" paint systems. Unfortunately the culture of many boatyards stems from slapping bitumen on in any conditions which they generally get away with. That's not going to end well with most epoxies.

2 minutes ago, Arthur Marshall said:

 Next time it came out it was done with epoxy and I am pretty sure it wasn't blasted right back to the metal first (I certainly didn't pay for it) 

 

With respect Arthur, it doesn't sound like you were there or know exactly what was done? It could be that you're right and the steel wasn't properly prepped but the paint all stayed on, however that doesn't mean that would happen every time on different boats painted in different yards. One or two anecdotes don't really mean all that much.

 

The point is that the technical data sheets from the paint manufacturers exist and should be read, understood and followed by anyone applying epoxy.

9 minutes ago, Arthur Marshall said:

I did wonder if that had happened to mine - the bitumen being on mill scale and it mostly got knocked off with the power jet when epoxing.

 

Could be. There are so many variables it's difficult to know. When my boat was gritblasted the mill scale was on so firmly the blaster didn't get all of it off. I ended up painting over it and it's never come off (the scale).

[captain flint]

1 hour ago, Paul C said:

https://www.smlmarinepaints.co.uk/uploads/files/library/files/DATA_SHEETS/Jotamastic_90_AG.pdf

The bit where it says something about depending on the durability required seems a little vague! 

Both boat yards I mentioned - who do very good 2 pack applications - insist that you just don't get a satisfactory key without shot blasting. But that doesn't mean they're right, and of course there are many products out there - even if they're right, they can only speak for those products they've used. 

[blackrose]

43 minutes ago, captain flint said:

Both boat yards I mentioned - who do very good 2 pack applications - insist that you just don't get a satisfactory key without shot blasting. 

 

I've epoxied over small areas prepped with power tools (back to bare steel) and the paint adhered fine and stayed on. In fact thinking about it I did the floor of my gas locker (widebeam) and a couple of other lockers which are subject to water ingress and 8 years later they're still fine and look as good as when I painted them. But in general what these boat yards are telling you is correct. Blasting is best because you get that nice keyed surface.

Edited March 4 by blackrose

[blackrose]

2 hours ago, Tonka said:

https://www.smlmarinepaints.co.uk/products/Epoxykit65

 

Can be used without shotblasting

 

I think I'd want to know who actually manufacturers this product. My understanding is that SML is a paint supplier rather than a manufacturer.

 

Whatever they say about it in the advertising blurb, the bottom line is that an entire coating system (including in this case the sanded back bitumen), will only ever be as good as its weakest layer.

[ditchcrawler]

6 hours ago, captain flint said:

And I'm not just talking below the waterline (although I do also know someone who bought a brand new boat by a pretty well known maker whose boats go by his name, where the 2 pack epoxy below the waterline started flaking off within a year). 

 

 

 

My day in Court with Collingwood Boat Builders – Retirement with No Problem

[blackrose]

16 minutes ago, ditchcrawler said:

My day in Court with Collingwood Boat Builders – Retirement with No Problem

 

They might do some right as I've seen Collingwoods older than that and the epoxy looks ok, but clearly their prep and possibly application is bad on others. Budget boat so I guess it's to be expected that they're not all painted properly.

 

I had my Liverpool Boat out after a year from new as I'd heard the blacking wasn't done very well (how do you balls up simple blacking?) When I hit it with the pressure washer the blacking fell off in sheets. However, if a boat's been epoxied one would hope for better.

Edited March 4 by blackrose

[Andyaero]

I've heard anecdotal evidence of Collingwood new build's epoxy peeling off in sheets after 2 years.

My new build will be shot blasted to remove mill scale prior to being epoxied, I think it has to be, to get Jotun's full warranty.

[Arthur Marshall]

6 hours ago, blackrose said:

 

.

 

With respect Arthur, it doesn't sound like you were there or know exactly what was done? It could be that you're right and the steel wasn't properly prepped but the paint all stayed on, however that doesn't mean that would happen every time on different boats painted in different yards. One or two anecdotes don't really mean all that much.

 

The point is that the technical data sheets from the paint manufacturers exist and should be read, understood and followed by anyone applying epoxy.

 

Could be. There are so many variables it's difficult to know. When my boat was gritblasted the mill scale was on so firmly the blaster didn't get all of it off. I ended up painting over it and it's never come off (the scale).

I don't think Red Bull yard was set up for shot blasting, and if he had I'd have expected to be charged for it, or at least it to be mentioned. As I said , it's anecdotal. If I'd been asked, I'd have had it shot blasted first - I was expecting it to be done with bitumen, so the epoxy came as a pleasant surprise, as did it lasting.