Why remove antifouling?

[MM5AHO]

There's been several threads about methods, speed, tools for and other aspects of removing antifouling.

But why remove it? What are the reasons for removing stuff that cost a lot to put there and cannot do its job after being removed?

 

[noelex]

If the coating becomes too thick it tends to crack and peel.

The biocide in antifouling leaches out first, so old antifouling (especially hard) is not very effective.

The biocides leach both ways so a thick layer of old, worn out antifouling tends to reduce, not enhance, the effectiveness of the new stuff.

The ideal is remove the old stuff before applying the new coats, but in practice a gradual build up is usually seen with the harder antifoulings.

Scraping the antifouling in the water is a way to remove growth and expose a fresh layer of biocide, but too much can be counterproductive.

 

[jwilson]

If the coating becomes too thick it tends to crack and peel.

The biocides leach both ways so a thick layer of old, worn out antifouling tends to reduce, not enhance, the effectiveness of the new stuff.

Hadn't thought of that: it might well be true.

Mainly though I keep AF thickness down as a smooth bottom is definitely faster. And I don't race these days, just like to sail efficiently and fast.

 

[neilf39]

After a given time the active contents will have leached out to a point it does no good. If you keep over-painting, at some point you end up with a very thick coating and what tends to happen is the lower layers degrade lose adhesion and flake coming off in bits and pieces taking anything painted on top with it. Your nice smooth hull eventually will look like the surface of the moon and have patches with no useful antifoul. To get back to a reasonably smooth surface and good surface to work from you then have an even harder job of scraping off thick layers of the stuff that scrapers and antifoul remover have trouble penetrating. the stuff that does remain also seems to be super hard and welded on.

I guess for the average cruising person a bit of a lumpy bottom isn't the biggest of their concerns but for a racer I think they want the smoothest, cleanest, most slippery bottom they can have. Comes to a balance of how often you want to do it and how much effort you think it's worth.

It's useful once in a while to remove it all back to see what's going on underneath and get back to having a decent finish on a well keyed surface.

 

[neilf39]

I can't type fast enough, beaten twice!!!

 

[awol]

But why remove it? What are the reasons for removing stuff that cost a lot to put there and cannot do its job after being removed?

I'd leave it there, Geoff, then you can use your rough arse as an excuse for any racing losses you suffer, for example, in the third week of May!

Removing sounds like overkill, flatting it down gives the new coat a key and allows you get up close and personal with any blemishes, blisters, pockmarks, rampant osmosis, etc. while you cover everything with a colourful slurry.

 

[Concerto]

Removed my antifouling so the hull could be epoxied. I know it had been done in the past so was not difficult to do.

 

[Hydrozoan]

About two years after we bought the boat, when she was about 12 years old, very bad flaking occurred and we had the hull blasted, presuming that antifoul had never been removed. We applied epoxy followed by primer, and have applied two coats of eroding antifoul annually ever since.

Some 9 years later, there is virtually no sign of flaking, paint adhesion seems good without extensive rubbing down and antifouling efficiency seems unchanged by any ‘inward’ leaching. We have kept to the same manufacturer’s products throughout, and the same antifoul, so if it does flake we will know it’s not incompatibility of layers.

But I do now wonder every time she comes out if that happy state of affairs has continued. How long do others find it takes for bad flaking to occur?

 

[Spuddy]

I recollect that one of the old school journos on a sailing comic said he kept slapping it on until there was a ledge wide enough to help him clamber aboard.

 

[cryan]

Many antifoulings are self polishing. This means that they constantly shed layers of itself when underway. If you paint new anti fouling over the old stuff you may as well just pour it in the harbour as it will not adhere to the substrate of the old stuff and will wash off much quicker than it otherwise would painted on a properly prepared substrate. --Or at least that's what a salesman from Jotun told me?

 

[reeac]

I've been applying one coat per year since 2003 plus a second on leading edges etc. with no flaking problems so far and not too rough a finish. The hull gets pressure washed at lift-out plus scraping off any residual barnacles during the Winter.

 

[MM5AHO]

I've been doing some experimentation on this over the past couple of seasons.
I accept that a smooth hull is an advantage for speed, and that for those very keen on racing, this might be a plan worth following.

But I dispute the following assertions:
* The biocides leach out so that the under layers don't perform any useful function
* The thicker the residual layers, the less adhesion the new coat has, and this leads to cracks and flaking, and uneven prevention of growth.

It is true that the surface eventually ends up like a lunar surface. And this must be that some parts have lost adhesion. But there's no visible difference in the growth. Its not possible to tell if a top layer has parted away, as what's exposed just looks the same, having used the same colour. It might be an interesting experiment to use another colour one year (but I've already bought this years, so not in 2015).
The hull is pressure washed every year, and I've sometimes done it again during the winter. Some poorly adhered material comes off. What doesn't come off must be well adhered.
I tried removing the old layers, but it was hard work. I used an electric scraper (the Lidl variety pointed to on these forums some time back). It was hard enough work that I did only one side. I then applied fresh coat of AF for the season.
There was a small difference in the prevention of growth. The side where I removed the old layers had slightly more growth. In other words, the old layers seemed to be enhancing the antifouling effect.
I did not notice that the boat went in circles, so water resistance didn't seem noticeably different. Of course it could be that at the slow speeds Contender does, it makes no difference anyway?
So I concluded that leaving the old antifouling on improves the performance in terms of preventing growth, while (in theory at least) it gives more resistance through the water, so perhaps slowing the boat by an un-measurable amount. But it saves the cost and effort of removal.

 

[Hydrozoan]

I've been applying one coat per year since 2003 plus a second on leading edges etc. with no flaking problems so far and not too rough a finish. The hull gets pressure washed at lift-out plus scraping off any residual barnacles during the Winter.

Thanks. Much the same as us - also on the East Coast with a similar boat - except we do two rollered coats plus a bit more on leading edges and waterline etc. (no boot top). No barnacles, just slime and very modest algal 'beard' at waterline. So perhaps it's time to drop to one coat ...

 

[charles_reed]

Best not to slap on such an excess that it needs to be scraped off. I use a single coat (except on high erosion areas) of different colours of self-eroding antifoul.
Now, after 9 years of use I sometimes am back to the original hard-racing antifouling put on over the 5 coats of gel-shield.
Having said that I probably do x3 the distances of the average cruiser.
It may be a matter of opinion, or not, that a thick underlay of old antifoul reduces the biocide available, but most of the manufacturers' technical departments seem to agree that a build-up of old is contra-indicated.
In truth, the only way to avoid fouling is to use the boat.

 

[GrahamM376]

But I dispute the following assertions:

* The thicker the residual layers, the less adhesion the new coat has, and this leads to cracks and flaking, and uneven prevention of growth.

On current boat, the old layers were very thick and flaky to the point where as soon as new antifoul was applied, patches of the old stuff would come off leaving lumps attached to the roller. No option but to scrape and, like others, took the opportunity to epoxy before applying new.

 

[Uno]

I have been talking to a few of the main antifoul producers since the end of last season (another story) and they all say that even the best of the self polishing products eventually leave a surface layer that is not as good as it was and that will then start to insulate the better bit underneath.
So i think the old layer still has good stuff in it but it just cant work as well as it originally did, one suggested that as long as the old layer is thick enough I should jet wash hard directly on lift out while the layer is still wet (this is easier said than done as many of us are lifted out together), if done hard enough it will remove the not as good insulating surface and what is left should work much better. The only warning i was given was to make sure I have enough product left on the hull to last the next season and not just the colour from what was there.

 

[Fantasie 19]

I've been applying one coat per year since 2003 plus a second on leading edges etc. with no flaking problems so far and not too rough a finish. The hull gets pressure washed at lift-out plus scraping off any residual barnacles during the Winter.

+1

 

[alan_d]

Many antifoulings are self polishing. This means that they constantly shed layers of itself when underway. If you paint new anti fouling over the old stuff you may as well just pour it in the harbour as it will not adhere to the substrate of the old stuff and will wash off much quicker than it otherwise would painted on a properly prepared substrate. --Or at least that's what a salesman from Jotun told me?

Salesman's bullshit, I suspect. Eroding antifoul is eroded by the passage of water, as you yourself say "when underway". If you overpaint with the same or a compatible antifoul, the under layer will stop eroding until the top layer has gone. If that were not true it would be pointless to apply more than one coat.

 

[Hydrozoan]

So, in summary, the knack to avoiding flaking is to match the rate of application of erodible antifouling to the rate of erosion, and that will depend on usage of the boat and where it is kept (presumably some erosion will occur on a tidal mooring, for example, as well as when underway).

I see that charles_reed's use of different colours helps in judging that, but we - like most people I see - stick to the same colour, probably for aesthetic reasons or out of habit. It also occurs to me (very obviously, I concede!) that alternating one and two coats in successive years could allow 'fine tuning' of applications if two is too many but one insufficient.