after grit/soda blasting

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oldvarnish

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I've got quite a build up of old antifouling and I want to have it blasted off.
A boatyard is telling me that I need to follow that up with filling, fairing, epoxy coatings (for osmosis protection) before going to primer and antifouling.

Is this guy having me on?

I thought that blasting was controlled in such a way that it removed the old antifouling and nothing else. I can't believe that everyone who has their boat blasted then has all the treatments he's recommending.

(A recent survey showed the hull to be 'bone dry' so no osmosis present)
 
No hint of the boat - so I'll post with what I've heard ...

Everyone who's been blasted finds they have tiny pin pricks in the gelcoat afterwards - this is probably to do with the layup of the gelcoat and nothing major to worry about, but (I believe) they should be filled - easiest way would be an epoxy coat I think.

boat yard isn't wrong (IMHO) ... and I don't think I'd go for blasting on the hull ...
 
I have just had my hull grit blasted. You are right in a way, it should only remove the paint and leave a surface ready for recoating. However, as we discovered you don't know what the hull is like under all that crud so it may need just filling pinholes etc, but you may expose more. Much depends I think on the operator.

My hull is predominately Cascover sheathed with some unsheathed wood under water. Discovered the sheathing was done at two different times (I knew that) but with slightly different processes which was not obvious until the paint came off. Discussion with the blasting company and a couple of phone calls to specialists and know what to do with it now (when it stops raining at times I want to work on it).

Don't know where you are, but Symblast did mine and they also do or sell coatings.

Hope this helps.
 
If I didn't go for blasting then it has to be a mechanical scrape ... bummer and a lot of hardwork - may be less painful to blast!
 
When you have had a boat for 30 years you get up to all sorts of things. About 20 years ago, in conjunction with the then builder of other Griffiths boats, I redesigned the keel and rudder deepening it by just over 300 mm, filling in between the keel and skeg and doing away with the bilge keels. The new deadwood is Douglas Fir - whopping great baulks, built up in my garage and inserted between the old deadwood and the ballast keel. Originally sheathed in epoxy glass cloth but it failed so I stripped it all off and just straight epoxy. Cleaned up well and delighted to see all my gougings when I took off the sheathing are still there! Reminder of the week I spent trying to fair it up!

So after consultation, the not so well done sheathing needs epoxy rolling in, the original well done bit has mostly intact original vinyl coating that not even a belt sander will move just needs priming and the bottom including iron keel two coats of Primocon. Had to close my eyes this morning signing the slip for the materials - but it is intended to see me out! (Post Bavaria)

BTW if you know anybody who wants a pair of bilge keels and a barn door rudder they are currently doing good service as garden ornaments.
 
No, the Bav may be next. Coming out tomorrow to have a high pressure clean to remove a green beard about a foot long that grew in 7 weeks on a swinging mooring. Bottom is a bit ropy after years of Greek AF and the keel lumpy. Having seen how well the Eventide was cleaned I may do it if I keep the boat. £500 approx plus the usual contribution to Mr International - but may be tempted with Coppercoat.
 
oldvarnish said:
I've got quite a build up of old antifouling and I want to have it blasted off.
A boatyard is telling me that I need to follow that up with filling, fairing, epoxy coatings (for osmosis protection) before going to primer and antifouling.

Is this guy having me on?

I thought that blasting was controlled in such a way that it removed the old antifouling and nothing else. I can't believe that everyone who has their boat blasted then has all the treatments he's recommending.

(A recent survey showed the hull to be 'bone dry' so no osmosis present)
Click to expand...

There is another type, Slurry blasting which is done I think with a wet pumice. This is what is recommended for removing anti-fouling, not Grit blasting which is very aggressive and will effect the gel coat, especially in careless hands.

I had my Westerly Oceanlord Slurry Blasted 2 years ago so that I could put on International Gellshield, but the finish left after blasting was not full of pinpricks, and it would only have needed a primer and antifoul to get her back in the water. I am therefore not sure what your boatyard is talking about.
 
Chris_Robb said:
There is another type, Slurry blasting which is done I think with a wet pumice. This is what is recommended for removing anti-fouling, not Grit blasting which is very aggressive and will effect the gel coat, especially in careless hands.

I had my Westerly Oceanlord Slurry Blasted 2 years ago so that I could put on International Gellshield, but the finish left after blasting was not full of pinpricks, and it would only have needed a primer and antifoul to get her back in the water. I am therefore not sure what your boatyard is talking about.
Click to expand...

I don't know you Chris but do you want a job?!!

You are right of course.

It isn't unknown to get a few pinpricks which are easily, locally filled. But if the boat is full of pinpricks the treatment was too aggressive.

But back to the OP.

Epoxy is many many times more waterproof than gelcoat. Once you are back to bare gelcoat it is an ideal opportunity to epoxy your hull and keel further water ingress out.

And if you buy a new boat you are, in my opinion, daft not to epoxy coat it. Water starts passing through gelcoat the day you put it in the water.
 
soda blasting

I just had my ferro cement hull soda blasted and also the fibre glassed decks. Fantastic in my view, I did also get a patch test grit blasted. The result was noticably different. I can believe completly that the soda blasting left the cement untouched (the finish was marble like) the grit did however leave a key.
 
I have just been reading your posts concerning the blasting of hulls to remove the antifouling and would like to offer my findings based on 13 years experience.
GRP hulls very much vary in quality of the gelcoats, this is between manufacturers and indeed individual hulls from the same manufacturer, gelcoat hardness varies as well as thickness.
For example Bavaria's seem to be very good with good, sound gelcoats with little or no air voids-however a very few I have done have been poor with prolific air voids-fortunately mine was very good.
Some manufacturer's hulls I very much don't like to blast because I have repeatedly found their gelcoats to be really bad-necessitating in the need to epoxy fill and epoxy coat after the blasting. Some people believe that it is better to find these voids and then fill them and then seal the gelcoat with epoxy and I agree to an extent but when you are stood next to a hull on an expensive yacht that looks like a piece of bread it is hard to say so to the poor owner.
In conclusion the blasting of a hull must be done as carefully as possible but also as quickly as possible to minimise the amount of time the gelcoat is subjected to blasting, the correct blasting abrasive is very important as it needs to be a well rounded and not overhard -many abrasives that are used are quite angular and will in my view quite badly pit the surface of the gel-some blasting companies will say this is a perfectly keyed surface and ready to paint-personally I feel a smoother finish requiring a light sanding to key to be preferable as this means the actual gelcoat has not been overblasted.
I hope you find this helpful and it doesn't put you off having your hull blasted to remove antifouling-I have seen some horrendous damage done to a hull by scraping and at present chemical stripping appears to be pretty ineffectual and inherently dangerous.
 
Oh by the way the type of blasting-slurry or grit, Grit blasting sounds more aggressive and it is, this method is used for iron keels and boats that have had their gelcoats removed as part of an osmosis repair treatment, grit blasting can be done wet (as a slurry) or dry.
Slurry blasting is a wet method of blasting and is no less aggressive than a dry method-as I stated above the most important thing is the correct abrasive.
 
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