Soda, slurry or grit blast?

[Massey]

Two years ago I removed all antifouling and stripped the bottom back to a Gelshield layer that had been applied some years before. I keyed the surface meticulously and applied 4 coats of epoxy primer (Hycote-152 aka ME-100) and then 6 coats of Coppercoat. After the first season the hull had developed 100s of osmotic blisters between the original Gelshield and the new epoxy primer. After the second (this year) I have many more 100s. A surveyor has examined the blisters and found them to contain a mildly alkaline solution and believes them to be due to incomplete curing of the epoxy. Beneath some of the blisters there is evidence of the Gelshield breaking down. The hull is dry with good moisture readings.

My plan now is to remove all layers (10 epoxy-based layers plus several Gelshield layers) back to the gelcoat, avoiding damage to the gelcoat. Can anyone make any recommendations as to whether soda, slurry or grit blasting would be appropriate? Also whether I should then be able to merely prime and anti-foul or should I build back up with Gelshield 200. I am reluctant to epoxy if not necessary due to the need to dry out/hot-vac and associated cost.

Any advice would be appreciated!!

 

[markdj]

Which epoxy has not cured? is it the primer or the coppercoat epoxy? IS there a website for Hycote with curing information? what was the weather/temperature when applying hycote?

 

[Elessar]

You have quite a challenge here. The problem you have if you use any type of abrasive blast is that the epoxy is probably going to be harder than the gelcoat. So as soon as you get a little patch through the epoxy the gelcoat could be destroyed in that patch before the bit of epoxy alongside it is completely removed. Maybe you could have it peeled off (we can't do that before anyone accuses me of a commercial post) although if the epoxy isn't cured you may be able to abrade it off, you may persuade someone to do a test patch but make sure you're there when they do! Or you could use an abrasive blast and ensure the operator doesn't attempt to remove all the epoxy, going gently, stopping when the gelcoat appears and removing the remaining epoxy with a sander. Whichever way this is not straightforward.

I'm really curious about why the epoxy didn't cure. Did you apply it? Did you measure the temperature and humidity beforehand? Could the mix ratio have been wrong? Could the mix have been incomplete? Who supplied it? would like to know more.

 

[seadog30]

Grit Blast

Hi

You will need to grit /sand blast, the other types would take an age to remove the old epoxy.

I had my 30 foot hull blasted back to gell coat this year. To remove back to gell is not easy and it needs a skilled operator, dont even think about a diy job.
I would recomend Symblast who did my boat, excent job done, they left a surface that was consistant and ideal for recoating.

Symblast can be found on the interweb at www.symblast.com

Good luck

 

[Bosun Higgs]

I had the same problem with an epoxy coat - in this case the West system applied by an east coast yard. The blisters were the reult of amine blush, the formation of waxy deposits due to cold and damp which were not removed before applying the next coat.

The yard removed the epoxy by gel planing, re-applied it and there were nop more problems.

Given that you are removing a hard coating on a softer sub surface, I wouldnt go near a blaster.

The yard paid for mine since it was their mistake. But in your case I wonder what would happen if you simply left it for a few years and then sanded off the top of the blisters and epoxied over that. I'm not convinced you need to remove it all so in your place I would be wanting to talk to someone in the paint business who really knew. I'd talk to "Passion for Paint" - googl eit. The guy there is ex International and consults on paint matters.

 

[gardenshed]

I witnessed a Farrow blasting unit in operation recently and the operator claimed that it has the control to clean inkjet print off from paper ..............
I'd like to see that, but the message is that the more sophisticated blasters have very good control so call up e.g. marine blast (http://sailingstuff.wordpress.com/marine-blast-services/) and ask them

 

[PeterGibbs]

Two years ago I removed all antifouling and stripped the bottom back to a Gelshield layer that had been applied some years before. I keyed the surface meticulously and applied 4 coats of epoxy primer (Hycote-152 aka ME-100) and then 6 coats of Coppercoat. After the first season the hull had developed 100s of osmotic blisters between the original Gelshield and the new epoxy primer. After the second (this year) I have many more 100s. A surveyor has examined the blisters and found them to contain a mildly alkaline solution and believes them to be due to incomplete curing of the epoxy. Beneath some of the blisters there is evidence of the Gelshield breaking down. The hull is dry with good moisture readings.

Any advice would be appreciated!!

Your description begs questions: you say the blistesr are not in the hull, but at the end state the Gelshield is breaking down in places, suggesting the blisters are backing into the hull. Can this really happen? This is important - if the blisters are "superficial" I would not revisit the Gelshield or risk the gelcoat with blasting. Treat topically.

I have never heard of Gelshield breaking down, having applied it to my last two boats and had no problems or symptoms for 9 years and 11 years respectively.

It is for Coppercoat to come up with a reason for the apparent instability of their preparation - is there any incompatibility in the layup you have used?

PWG

 

[ccscott49]

Anybody know if these services are available in Spain, or Corfu (Ionian)

 

[Bosun Higgs]

I witnessed a Farrow blasting unit in operation recently and the operator claimed that it has the control to clean inkjet print off from paper ..............
(http://sailingstuff.wordpress.com/marine-blast-services/) and ask them

I used Farrow for my last boat - the one after the one with amine blush. It's a good system and did an excellent job. But it doesnt get round the problem described by Elessar. No blasting system can do that.

 

[Elessar]

I witnessed a Farrow blasting unit in operation recently and the operator claimed that it has the control to clean inkjet print off from paper ..............
I'd like to see that, but the message is that the more sophisticated blasters have very good control so call up e.g. marine blast (http://sailingstuff.wordpress.com/marine-blast-services/) and ask them

we have 2 farrow machines, but still see my caution in my post above. This could be a tricky one. If the coating is harder than the substrate you have a problem. In this case it could well be, ME100 properly applied is 400 microns thick and very durable. Care is required.

 

[Foxy]

If a pro says its a tricky job, I'd be more likely to believe him......

 

[Massey]

coppercoat blisters

Thanks everyone for their replies. To answer a few questions: the conditions when I applied the epoxy primer and the coppercoat were perfect - breezy, no condensation with temp 14 degrees. The preparation was meticulous - I new this would be key (excuse the pun!), it was mixed very thoroughly to the manufacturers instructions and application was to the letter. There was no amine blush and it all appeared perfect until haul out 10 months later. It is interesting that where I popped the blisters and re-applied epoxy primer plus coppercoat last year, new blisters have formed again this year. The gelshield under the blisters is slightly powdery and the fact that new blisters formed in the same place implies that it is the condition of the gelshield (in localised spots) that is causing the problem.

 

[Pasarell]

Blisters

In your original post you say the Gelshield was applied some years before the latest epoxy and Coppercoat. Assuming there were no problems before it is highly unlikely the Gelshield is the cause of your problem
In your latest post you state the conditions were "perfect" when you applied the last series of products.

It seems to me there are a few possible scenarios.

1. The Gelshield surface was contaminated in some way before you coated it. How long did you leave it after stripping previous coatings before the next coat of epoxy? Did you do a final clean just before applying your 1st coat of epoxy? Gelshield (and most other epoxies) are not UV resistant and will chalk if left exposed for any length of time - especially if they are already old. That chalking will give a powdery surface that will probably cause blistering in subsequent coats unless removed.
2. If you did not clean just before application of your first coat and stripping of old coats had not just been completed there was likely to be some accumulation of grease and dust on the surface which would cause lack of adhesion of your 1st coat. This is likely to be first seen as blisters.
3. You state the conditions were perfect at 14C when you applied your products. I always find that is quite a problematic temperature - high enough for most epoxies to cure but likely to vary if left overnight. Amine bloom occurs often if temperatures vary during the curing process. 14C during the day often means 7 or so at night which will almost certainly cause a bloom to happen. You can't see, feel or smell amine bloom so have to assume it is there, and remove it, before applying the next coat if there is any doubt at all.

Soda, slurry or grit blasting?
Assuming your Gelshield is actually OK and the problem is with the subsequent coats it seems to make sense to go back to the Gelshield as a first step. This gets over problem of blasting hard epoxy off to a softer gelcoat. As there are lots of recurring blisters at that interface too it should form a relatively straightforward stopping point. The epoxies you applied will break away from the Gelshield layer under impact from blasting. Not, unfortunately, completely cleanly but enough to leave a surface you can prepare by hand quite easily.
If blasting I would go for slurry. Grit is too aggressive and can suddenly go further than you want. Soda will be very slow on multiple layers of epoxy and end up costing a lot. Slurry, in the hands of a good operator, is a good compromise. Slower but more controlled than grit, faster than soda. The problem with blasting is usually the person holding the nozzle and in your situation I would give him a very thorough briefing as to what I wanted, then watch every move - no matter how much he hates that! Probably a better alternative in your situation would be to peel if that is available and affordable.

By the way I love the comment about cleaning ink off paper with soda blasting. Strictly true, but not with the kit they will use on your boat! Blasting is hard, dirty, physical work. Despite the claims very few people do it with care and finesse!

In summary it seems you have 2 choices assuming the blisters are between Gelshield and subsequent epoxy. Carry on using the boat and ignore the blisters until any problems become bigger which probably means detachment back to the Gelshield or clean off all the epoxy you applied and rebuild using your chosen scheme.

Whatever you do it will be interesting to hear how you get on.

 

[vyv_cox]

Anybody know if these services are available in Spain, or Corfu (Ionian)

There's a small grit-blast business inside Gouvia Marina. Seems it may be a British operator but I never managed to catch him in, although only there for two nights.

Pro-Blast - Grit Blasting Service
Landline: (0030) 26610 91056 Mobile: (0030) 6932 536126
Gouvia Marina
Kontokali
Corfu
49100
Greece

Welcome to Pro-Blast, where we provide a specialist grit blasting service covering Corfu and other parts of the Ionian Islands. The results speak for themselves - please see photgraphs below which ... MORE»

Marine applications:

- Rust and paint removal
- Anti foul removal
- Re epoxy osmosis treatment
- Rust keels cleaned
- Rusty boat trailers

Available throughout the year, please contact us on the above contact details for a free quotation. Thank you.

 

[LeonF]

Your description begs questions: you say the blistesr are not in the hull, but at the end state the Gelshield is breaking down in places, suggesting the blisters are backing into the hull. Can this really happen? This is important - if the blisters are "superficial" I would not revisit the Gelshield or risk the gelcoat with blasting. Treat topically.

I have never heard of Gelshield breaking down, having applied it to my last two boats and had no problems or symptoms for 9 years and 11 years respectively.

It is for Coppercoat to come up with a reason for the apparent instability of their preparation - is there any incompatibility in the layup you have used?

PWG

Peter when I bought my boat there were some wet spots which the surveyor attributed to the Gelshield breaking down. He said it may result in osmosis in time, or not. It was impossible to predict. I have noticed some flaky anti-fouling in the past, in isolated spots. This year scraping them showed up the green Gelshield layer. Do you know what the best primer to use might be ? Should I sand back hard to get a key ?

 

[Elessar]

Soda, slurry or grit blasting?
Assuming your Gelshield is actually OK and the problem is with the subsequent coats it seems to make sense to go back to the Gelshield as a first step. This gets over problem of blasting hard epoxy off to a softer gelcoat. As there are lots of recurring blisters at that interface too it should form a relatively straightforward stopping point. The epoxies you applied will break away from the Gelshield layer under impact from blasting. Not, unfortunately, completely cleanly but enough to leave a surface you can prepare by hand quite easily.

this is absolutely true - I did suggest a test patch to see if abrasive would work. In fact properly applied gelshield will have 3 (or more) different coloured layers so it would in this case be possible to get all the outer epoxy off and be sure you're not going too far through the gelshield.

But..........

We see gel shield type things that are no thicker than a coat of paint and with a poorly prepared hull underneath and so I would always be suspicious of it - if properly applied gelshield (multiple coats 100+ microns thick per coat) is in perfect condition why would you spend £100s on another layer of epoxy on top?

It could be possible to do it perfectly with an abrasive blast if the gelshield is thick and sound but it remains one I would approach with caution, if I was the owner I'd ask for a test patch and check I was happy before allowing them to proceed.

 

[Cariadco]

I can personally recommend Pro-Blast operating out of Gouvia Marina in Corfu.
Martin sand blasted my mast, back to the bare metal, and it was superb.
It was a shame to paint it!!

Regards,