rothwell_neil 9:17pm, 30 January 2009
Information direct from top paint guru. Get back to clean metal if possible, if not then as good as you can get, shot blasting best way but most of us don't have access to this! Then two coats of two pack epoxy, recommendation for H Marcel Guest Epilife paints available in 1 litre pots. Follow temperature and overcoating instructions to get good bond between coats. Epoxy needs to be cured at correct temperature to get rid of solvent and to cure. Also needs to be overcoated within set time period to get inter coat adhesion. If going to leave and forget for a while then can improve coating performance by tapping hole and fixing small zinc sacrificial anode to keel. Do this after painting and just screw to keel. It will last a long time as will only protect any small defect and not corrode rapidly.

www.hmgpaint.com
www.hmgpaint.com/content/datasheet-pdfs/tds_hmg/35.pdf

guillainevib there is indeed a university department in Manchester that only does post graduate MSc and PhD qualifications in corrosion. Manchester may not be near the sea but it knows all about corrosion. This is the only corrosion department in the world and as it puts out about 50 MSc and 20 PhDs per year has led to a Manchester Mafia that specialises in corrosion all over the world. Great for getting sailing pals, one of my old mates has just bought a 41ft Dufour and based it on the Gold Coast of Australia. This is probably why I have sailed or windsurfed in most parts of the world including Alaska and Far East, the cold water in one area and sea snakes in the other do wonders for your gybing skills on a windsurfer.
Skykomish E29 10 years ago
Thanks Neil for going to the trouble to find out about this.
Can I clarify that the Epoxy used is NOT the epoxy based paint used to prevent osmosis, but standard Epoxy adhesive like West or Blakes SP?
rothwell_neil 10 years ago
Epoxy is a type of resin and has different formulations for laying up glass, as an adhesive, osmosis treatment and painting. Changing resin types as well as ahrners can change the properties. The 2 pack paint would be suitable for osmosis treatment as well on a properly dried surface. This paint is used to protect the steel legs of off-shore platforms in the splash zone which is an arduous service.
Daddsie 10 years ago
VC tar is an epoxy primer, I used it last time with differing results. This time however I am having the keel blasted off.
rothwell_neil 10 years ago
Daddsie VC tar will work as a primer coat on clean steel. Epoxy is like most paints in that it benefits from a primer coat. The best primers for steel contain zinc and this actually works the same way that zinc anodes work in protecting any paint defect sacrificially. So when you see zinc rich primers they are designed primarily for steel.

I guess as I know this stuff I ought to tell you about epoxy paints, apologies to those that know already.

There are three things that are important to getting a good and that means long lasting epoxy paint on anything. Surface, temperature and over coating.

Surface preparation is really important, back to bare substrate be it GRP or steel is best. So for the keel shot blasted or belt sanded to bare metal is ideal. Clean from grease and any other contaminant and dry.

Conditions for application are key, the temperature of application is very important so that the paint can cure and also so that any solvents in the paint can escape, otherwise they will later cause problems and delaminate any subsequent paint layer. So warm enough for the initial cure, also not applied under high humidity conditions and not too early in the day when dew may still be around. However don't apply late in the day as the air temperature will drop below cure temperature and any dew will cause blooming of the surface. In other words if you can't do this in a warm dry indoor place then pick the day and get everything ready the day before so that can paint as soon as warm enough and get as long as possible in to cure before it cools down again. If you wait to the middle of summer can get two coats on in one day. It it says don't apply below 10C don't! Also make sure that it will be over that temperature for as long as possible.

The next thing is overcoating. Epoxy is a cured paint which crosslinks and polymerises as soon as the hardener is added. This continues for a period of time after the initial dry and if left too long will be difficult to bond to without abrading the surface first. This is because the surface will be fully crosslinked and further coats will jot be able to crosslink to this layer. Painting a second coat whilst the first is still curing will result in crosslinking between coats and thus get a really strong final coating which is why epoxy has such a good reputation as when applied properly you get one fully bonded coating rather than multiple layers.. So read the over coating instructions, if it says drying time, 1 hour touch dry, 16 hours firm dry and 3-7 days hard dry then overcoat next day or two but definitely before the week is over. Otherwise further coats will need surface abrading or they will peal off.

The problem most of us have is that we try to do the work when the weather is not ideal as otherwise the boat would be in the water and we would be sailing. So leave the painting as late in the year as you can and hope for a good weekend when you can get two coats on. When applied properly epoxy will give many years of protection without requiring any repairs.
cornel carpenter 10 years ago
Achilles has some 10 coats of epoxy on her below water, these were applied during a nice warm period in the summer of 2007.
She's abut to have another 2 coats applied this summer.
That's not to mention the epoxy that was used to fill, repair the damaged gelcoat and glass fibre on her hull, which varies in thickness of 3mm to 20mm in places.
The interesting thing thing for me was when I had her slurry blasted, what became patently obvious was she didn't suffer any osmosis anywhere.
Not bad for a GRP hull that was 36 years old, in that respect Butlers did an excellent jog of laying her up.
Now just to make sure, I put lots of epoxy on her, well you've got to make up for the cast iron she's lost of her keel.
Skykomish E29 Posted 10 years ago. Edited by Skykomish E29 (member) 10 years ago
Cornel,
This topic came up when we met Chris Butler in September, apparently Achilles have never had a problem with Osmosis. Chris attributed it to the fact that he did not add pigment to the gelcoat below the waterline, I would suggest that from the way he kept a tight reign on quality control the build quality my have also had something to do with it. Chris was saying that Surveyors like these boats due to there being so few problems attributed to them.... apart of course from yours, but as you state yours was one of the very first boats and perhaps was still in experimental stage.
Daddsie 10 years ago
Rothwell, thanks for that.

I stripped the keel last time back to bare steel, then gave it four coats of VC tar, numerous coats of primacom then anti foul, I still had patches of rust blistering through. The work was carried out in May.
busy home [deleted] 10 years ago
I did Chill with red lead and antifoul two years ago .. no probs so far
rothwell_neil 10 years ago
That shouldn't have happened. Bare metal primer coat (single coat) four top coats would give a total film thickness of 250 microns. (50 microns per dry coat) This would be expected to give a 10 year life in immersion in seawater. 85% of all paint failures are due to poor surface preparation. After blasting back to bare metal make sure you wash with fresh water as paint doesn't like salt crystals. Everything is against us painting on the hard next to the sea! If I had gone to this level of preparation i would also bolt a small zinc anode to the keel as this will protect any defects and stop the blistering.
busy home [deleted] 10 years ago
If you look at the pic of the gk24 ..... taken in September.... no weed or slime and no antifoul???
Daddsie 10 years ago
Fit an Anode!!!! all that drag, might as well tie a bucket to the back :)
rothwell_neil 10 years ago
For a properly painted keel the smallest disc anode would do But I guess you racers won't even eat lunch before you get on the boat to save weight!
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