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It’s a question many are faced with when it comes time to antifoul their boat. If you buy a brand new boat, these days most are what we commonly call fibreglass. In reality it’s generally polyester resin covered with a thin layer of white gelcoat. The resin is commonly a general purpose, cheap resin especially with imported boats. If you put this vessel in the water with nothing on it, moisture will eventually penetrate the gelcoat and resin and get in and start breaking down the glass reinforcement and styrene and becomes osmosis. This is a brown smelly substance that seeps from blisters that form on the Gelcoat. This is a chemical process that I am not going to cover here but if your interested read the booklet on it on the West Systems website. Covers the topic really well. Page 6 it starts.


Boats built with Epoxy resin are far less susceptible to getting osmosis.

The general rule of thumb with a fibreglass boat is you need a couple of coats of a high quality, high solids epoxy coating to try and waterproof your hull.

This is then followed by what is called a tie coat. This tie coat is designed to grab the epoxy coat and give your antifouling paint a good compatible surface to adhere to.

Think about this Jotun scenario;
Primer Coat to exposed glass 2 coats of Jotacote 605 or 2 coats of Jotamastic Aluminium. Both are around 84% solids. You need to key the Gelcoat with 80 grit so it’s nice and rough and it gives a good adhesion.
These coats need to be 8 hours apart. If it’s too soon the solvents soften the layer underneath and if it’s left too long the epoxy resin in the paint goes too hard, and you would need to sand it again before another coat can be applied. So apply within the recoat table on the spec sheet.

These two coats of epoxy primer are probably the two most important layers of paint on your boat as you are trying to stop moisture ingress which causes osmosis.

The next coat of paint, your tie coat can either be Vinyguard 88 a vinyl primer (single pack) or you can stay with a two pack system which is my favorite path, it’s called Jotun Safeguard Universal. Its a modified epoxy vinyl and the great part is it has a 2 hour recoat time. Just 1 coat required. It offers more water resistant qualities than a single pack vinyl.

So now we have 3 layers of primer on a new hull and we are ready for the Antifouling.

Jotun Sea guardian is the most common, and a very good antifouling. It has over 700 grams of copper p/l and although it’s not on the label it has 300 grams of zinc p/l as well. For a new antifouling job like we have described above, you would want 3 to 4 coats to give you enough film build to last 18 months. Of course this depends on moving your boat often. You can’t antifoul a boat and leave it on the mooring for 3 months!

Of course your primers are also to be applied with the correct film build. I think from memory it’s about 150 um Dft. So as you can see, it’s an easy process if followed correctly. Check the paints data sheet for correct film build requirements.

The next scenario is 2 years goes by, you have hit a few sandbanks grown a few barnacles where you knocked the antifouling paint off.

You come to The Boat Works and the team waterblast your hull upon haulout and because you used an ablative antifoul paint like Jotun Seaguardian, most of those 3 or 4 coats have worn away and the tail of it water blasted off.

You grab some 80 grit wet and dry paper and a hose, and you wet rub the whole hull area and running gear.

Great fun and if you used a blue antifoul you start to hear a few smurf jokes over your shoulder… Grab a strip disc and remove all your old Propspeed.

I do sell disposable dust suits to keep you a bit cleaner!

Putting that aside, you should now be back close to your original coat of Safeguard Universal. All nicely in tact and now keyed, except where you hit those rocks buried in that sandbank. Hopefully not after those few cocktails you enjoyed, that’s illegal I am pretty sure 🙂 But, you can see exposed Gelcoat or even a little resin or glass you can’t quite tell.

No problems, sand that area up and clean it well, wipe with some acetone and you head on up to Marine Trade Supplies and buy a tin of Norglass epoxy filler and a tin of Jotacote 605 epoxy primer. Mix the Jotacote 605 well and paint it on any bare glass patches and go home for the night.

Next morning you open that Norglass Norfill up and fill and fair any dings from your bottom encounters let it dry, then paint on a second coat of Jotacoat 605 right over those patches so it is sealed well and ensure there is no wicks of fibreglass poking thru the paint or filler acting like a straw where water can be sucked in. Note the Norfill is now even encapsulated by Jotacote 605. Nice!

After 6-8 hrs we should now be able to paint our Seaguardian over the hull and get a full coat on even the patches of Jotacote 605 we did in the morning. We go home again proud of our achievements!

It’s the end of day two on the hard, we are all primed and even have our 1st coat of antifoul applied.

Day 3 begins with our second coat of Seaguardian over the whole job nice and early. We are done by 9 am so slip over and put a couple of cold beers in the fridge you will need them later!

We spend the rest of the day tending to prop speeding the running gear.

Before we go home we put a third and final coat of Seaguardian on the waterline and all leading edges where there is high water flow leading to quick wearing of our paint or where we are tempted to scrub the waterline to keep it looking nice. Try to get 6 to 8 hours at 28 degrees between your antifouling coats and 8 hours prior to launch.

Prop speed is done, antifouling refreshed, go grab that cold ale to celebrate and go home.

Launch next morning around 10am after 8-10 hours at 25 degrees of drying time.

So the 4th morning we are back in the briney and we can give her a wash down and head off home! I prefer not to wash and wax a boat out of the water, you don’t want wax clogging your fresh antifouling..

There are of course variables with everything but this is a pretty typical scenario at The Boat Works and Marine Trade Supplies.

Aluminium legs is a big variable you would need Jotun Seasafe Ultra there not Seaguardian!!

Some professional applicators will apply the antifouling paint direct to the Jotacote 605 and skip the Safeguard Universal step all together but they are experienced. They will ensure the Jotacote 605 is cured enough so the solvents in the antifouling paint won’t soften the Jotacote 605 layer enough to leave tiny microscopic tracks thru to the gelcoat which, if not done properly leaves a track for water ingress.

I prefer two coats of that Jotacote 605 8 hours apart. It means the first layer on your hull is well and truly in tact and there is little chance of breach.

If paying an applicator, the old scenario of accepting the cheapest quote for your job where it’s done as quickly as possible so the applicator can get onto the next job, remember you do get what you pay for. If your having the job done for you, don’t push your applicator to the point where they have to skip coats. Understand their proposal and ask what coats will be applied. If you want more coats be prepared to pay for it and know what extra protection your getting for your coin. Plus, keep a logbook so when your recoating you will not have incompatability issues. Try to be present so you can see your layers and understand your boat better.

Paints are not paints either. Some epoxy primers have only 42% solids. So let’s think about that… A 4 litre can has 58% solvents in it!!! You would need to apply double the amount of coats to get to the quality of 1 coat of Jotacote 605 which is 82% solids or only 18% solvents.

Talk to your applicator and research your data sheets. The cheapest quoted antifouling job is not always what it seems. Most applicators are well versed and will do the job you want. They like giving you a quality job but they are driven by being competitive or they don’t get the job. Why is the other guys quote hundreds of dollars cheaper? Maybe they are skipping a whole coat of primer or only 1 coat of cheap antifouling..

I know quite a few quality antifouling applicators that will give you the job you want if your not dollar driven and expect a quality job for your pride and joy. No one wants to pull a boat out covered in osmosis blisters.

Come in and have a chat with me, I’ll be happy to point you in the right direction if you want to do it yourself or if you prefer someone to do it for you.

Thanks for reading!


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