What to do after blasting grp hull

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mikebutler

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I have just had my hull slurry blasted below the water line which has revealed that it has been epoxied and is in good condition (so a man told me). My questions is, what do I do now. Someone has said I should put 2 coats of Gelshield on before putting any antifoul on. If it's been epoxied, why put Gelshield on? I was thinking of just putting antifoul primer on top of the epoxy, followed by a couple of coats of antifoul. I am somewhat confused about Gelcoat, Gelshield, Expoxy, etc., anyone know of a good tutorial on these things?? Thanks.
 
It really depends on how good the underlying epoxy coating it. Just in case the slurry blast has damaged the epoxy coating it would be a good idea to but a coat or two of some from of epoxy barrier like gel-shield (there are other equally good products). Then prime and AF as normal.
 
The coatings manufacturers websites would be a good place to start. International yachtpaint's is www.yachtpaint.com/uk I am sure you can find others with Google.

There may be some written literature you can request. Telephone calls to the technical departments will get you the best advice, IMHonestO
 
G'day Mike,

Slurry blasting is a high skill and it's not at all hard to do 'some' damage. That said I would be taking a very very close look at what is left.

If I found any damage or very dull areas I would be looking at adding more epoxy, nothing sticks to epoxy like epoxy.

Then an epoxy primer followed by antifoul.

Slurry blasting leaves a finish clean enough to wash off, look for any glossy areas and give them a light sand with a 250 grit-just enough to take most of the shine off, wash again, wipe with acetone using lots of rags (Only white and cotton is best) just prior to adding more epoxy.

Apply epoxy with an 80 to 100mm roller and tip off with a wet long bristle 2 inch to 3 inch brush. You can avoid sanding between coats by applying wet on tacky. Temperature Must be better than 16*C and humidity below 73%. Do not apply any after 2pm or you run the risk of moisture entering the resin before it has skin cured.

After curing you need to remove the residue left by the curing process, just a running hose and fresh water (no chemicals at all) and a common plastic kitchen scourer, the scotchbright type, green on yellow foam. Start at the top and work your way down, just rub the scourer over the surface till the water stops beading into little balls, takes very little time or effort.

Sand only after cleaning or you will just spread the contamination. using a 250 grit to just take off the very high gloss finish, wash and dry, you are ready to apply the epoxy primer.

I Hope this helps.

Avagoodchrissie......
 
International Gelshield is the best known of these systems, and the instructions on the tin talk (when I last looked) about 5 coats to achieve critical micron thickness.

When you've checked this out at the chandlers/online, you'll have to make an assessment of how well your coating stands after the blast from hell. Likely you have lost the equivalent of a coat, assuming the original was up to spec (5 coats)

In any case it would not be a mistake to add a further coat to seal what's there, and prep the bottom for a coat of antifoul primer, before applying the antifoul.

I recall these have to be applied in fast succession to key into each other. Otherwise, you have to key the dried paint before the next layer is added.

When that's all over you will no doubt make the mental note, as we all do eventually, not to blast the underside ever again in pusuit of some elusive state of perfection!

Good luck,

PWG
 
out of interest was the the hull in a clean state after the blasting or had you prep work to do prior to paint applications.
the reason i ask is my hull was grit blasted & not slurry blasted as instructed / requested. i then has a lot of prep to do myself /forums/images/graemlins/mad.gif /forums/images/graemlins/mad.gif /forums/images/graemlins/mad.gif
 
The chap who did the blasting said the epoxy was in good condition and only required "a bit of a rub down" then he said to put 2 coats of gelshield on before antifouling it. He also said not to bother with the antifoul primer, just make sure to put the first coat of antifoul on a couple of hours after doing the 2nd coat of Gelshield.
I gather, from what others have said in this post, that Gelshield is a trade name. If so, what am I really looking for??
Thanks.
 
Nothing is totally waterproof including epoxy. However the degree of waterproofnes depends on the thickness, and since its likely that the abrasion will have taken some thickness off the coating its best to put some more on.

When I did the same exercise I put coats of Blakes solvent free on the hull and the last coat within the time window allowed (something like 24 hours) was the antifoul primer. I've heard of people putting the first antifoul on top of epoxy that hasnt quite gone off but whenever I have tried this on small areas of repair, the anti foul hasnt stuck.
 
i have an ongoing dispute with the marina. i still owe monies. i got a so called discount on topsides paint repairs but 1/2 the hourly rate & double the Hrs springs to mind therefore NO DISCOUNT @ all /forums/images/graemlins/shocked.gif
 
[ QUOTE ]
I gather, from what others have said in this post, that Gelshield is a trade name

[/ QUOTE ] If you look at the website I gave the link for you will find that "Gelshield" is indeed a registered trademark of Akzo Nobel and that Gelshield, Gelshield 200 and Gelshield Plus are two component solvent free epoxy coating systems marketed by International Yachtpaints for the treatment of hulls affected by osmosis

Ring Internationals technical department to get good expert advice!
 
I thought that slurry blasting was done when a decision was made to remove the gel coat. After which the exposed fibres are washed repeatedly for a reasonable period to remove the chemicals that created blistering and damage to the fibres. My further understanding is that this washing actually speeds up the subsequent drying allowing the application of epoxy layers when dry?
 
Slurry blasting by eg Farrow System can be used to take off just about any amount - antifoul, paint by the layer, or a hole right through the lay-up.

Repeated washing of exposed lay-up can be done to remove the glycol which causes water-retention and osmotic blistering. A "reasonable period" could be several pressure washes a day for several weeks, which is why HotVac is cost-effective.
 
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