Repainting a steel hull: how many coats of primer?

[BluJu]

Hi all,


the last time the hull of my steel boat was sandblasted and repainted was ~15 years ago (by the previous owner; i bought her 5 yrs ago). I have no serious rust issues but rather several recurring spots i'm getting tired of patching up every year so i'm (nearly!) committed to a full hull repaint job.


I'm seeking advice about the recommended number of primer coats below and above the waterline. It seems many folks agree on a minimum of 4-5 coats below the waterline but is there ground to those numbers? is it a matter of building a certain primer thickness? dependent on the primer used?



Similarly, at what frequency do steelboat owners blast and repaint their hulls? Or put differently, assuming a professional, quality job, how many years can I expect to get before having to worry about rust spots popping up?


thanks!

[coopec43]

I think I'd be getting professional advice form reputable paint manufacturer (but not necessarily one of the international manufacturers}

In Western Australia we have a local paint manufacturer who supplies paint to the offshore oil drillers so I'd be confident of their advice

Maybe a bit of a lead here?
https://www.paintsforboats.com/steel.pdf

https://www.cruisersforum.com/forums...at-152512.html

https://www.cruisersforum.com/forums...at-222613.html

[slug]

Originally Posted by BluJu

Hi all,


the last time the hull of my steel boat was sandblasted and repainted was ~15 years ago (by the previous owner; i bought her 5 yrs ago). I have no serious rust issues but rather several recurring spots i'm getting tired of patching up every year so i'm (nearly!) committed to a full hull repaint job.


I'm seeking advice about the recommended number of primer coats below and above the waterline. It seems many folks agree on a minimum of 4-5 coats below the waterline but is there ground to those numbers? is it a matter of building a certain primer thickness? dependent on the primer used?



Similarly, at what frequency do steelboat owners blast and repaint their hulls? Or put differently, assuming a professional, quality job, how many years can I expect to get before having to worry about rust spots popping up?


thanks!

15 years is a common interval

Each time you blast you remove metal , you don’t want to blast too often

By number of coats you mean paint film thickness

This data is supplied with the paint system

The system is two different paints

First is a substrate primer formulated to achieve maximum adhesion to the steel
Second is the water barrier applied over the substrate primer

Consult the literature for your paints

Paint is applied by film thickness

International Paints or Awlgrp are the paints I have used

Note that substrate primer is applied when the steel is still white ...freshly blasted ...only few hours permitted between blast and prime

Substrate primer should be applied with a spray gun
A roller is not permitted , the roller grabs the course blasted steel and leaves fibers behind

These fibers cause moisture wicking

Basically the film thickness of the substrate primer is thin , the film thickness of the waterproof barrier is thick

Google Awlgrip. Painting steel.... for a better description

Many different paint companies to choose from... ask a local what system they use

[Michael Cobbe]

To stop rust use Zinc rich epoxy for the first coat.

[MartinR]

In my opinion the best primer to use on a sandblasted hull is Intershield 300, if you can afford it;-). It is very hard wearing, abrasion resistant and protects very well.

4-5 coats would be plenty. I think the recommendation from International is 3 coats. It is important that the primer is applied by spraying. This basically goes for all primers if you want the coating to last. Rolling, brushing works for small areas and for brush ups. If you need to roll/brush, increase the number of coats.

[Gaia]

Originally Posted by Michael Cobbe

To stop rust use Zinc rich epoxy for the first coat.

Under the water line? We used it to great effect above but understood not to be used below. Above the line failure occurs, after many years, more than 15, at the zn and top coated interfaces in our case., not below the zn epoxy and is thus easy to repair as there is no rust. One then sees the zn hydrolysing, a slight whitish bloom, easily sanded and repainted.

I wonder if water diffusion under the line would accelerate this process hence the reason for not using it below. Check with the manufacturer as my information is old as is my memory.

Also if spots occur in the same area make sure there are no electrolysis issues.

Jim sv GAIA

[hpeer]

This is a thorny subject, especially the zinc rich epoxy under the water line.

I went through this myself and talked to a lot of people including Ameron representatives. Ask 10 people and get 12 opinions. My take away is there is no consensus.

I do put so the zinc primer. Many don’t, many do. With Ameron paints, and I suspect many others, you want to “hot bond” between coats, don’t let the first coat get 100% dry before putting on the subsequent coat.

I’ve been using Ameron 302 zinc and then 235 bar rust with good results. But prep is the main thing.

Then ask about using acid and that at will start another fight.

[Sailing August]

Consult your paint manufacturer. They will tell you.
1. Blasted surface texture
2. Chloride Test of the steel
3. Primer Coatings
4. Stripe Coating
5. Final coatings
6. Anti Fouling Coatings.
The paint manufacturers are very open and ready to respond technical advice.

[Stmurray62]

3 years ago, on a vessel I skipper, I supervised the bottom job of a 75 ft steel hull ketch. We sandblasted and then primed with a 2 part epoxy (PPG Americoat, about$100/gal). The last primer coat was just a bit tacky before painting with a 2 part epoxy bottom paint(again PPG Americoat). When we hauled her this spring all weneeded to do was lightly sand the bottom paint and repaint. Did not need to remove remaining bottom paint, nor re sandblast, It worked extremely well. The PPG marine paint I am describing is used by the USCG, Navy & Army. It does NOT have any metal content.

[captlloyd]

Originally Posted by Michael Cobbe

To stop rust use Zinc rich epoxy for the first coat.

One of the finest steel boatbuilders East of the Mississippi told me to definitely not use zinc rich primer on my boat. Zinc rich primer is very weak, it will actually split in two, another words it will adhere to the steel and adhere to the finish coat but will actually split in between. I’ve seen it happen, big sheets of paint just stripping off in the wind. The boat builder was George Sutton.

[oceanshoretiga]

I have had two steel yachts over a period of 39 years. In both cases I was advised to remove the zinc rich paint from below the water line (Sandblasted). It appears the zinc acts as an anode and so the boat is over protected (ie Cathodic) and the action of the zinc blows the paint off. I was having bubbling because of this.

My current steel yacht, Tiga, was built and painted professionally in Sydney about 1978 with zinc rich paint, following sandblasting, inside and out.k. As I said above, I had to sandblast below the water line, which I did, and no further problems from that source.

Tiga has the original paint above the waterline: deck cabin, inside and out, etc. Yes, I do have some paint failures and do have constant spot rust issues but most of those are caused by fittings and a lack of stainless steel welded fittings (common in the 1970s). Some are caused, in my view, by poor sandblasting and paining in places ‘difficult’ to get to.

I believe the boat was painted with Jotun paints with the top coats being what in those days was called chlorinated rubber paints. Never as glossy is epoxy, of course, but easy to use. I overcoat the topsides (no sanding, etc) with a roller in a few minutes every few years, sort of when I feel like it!

But, keeping steel good is still mystery around salt water, especially offshore.

Good luck, steely boat owners.

[David B]

Originally Posted by BluJu

Hi all,

the last time the hull of my steel boat was sandblasted and repainted was ~15 years ago (by the previous owner; i bought her 5 yrs ago). I have no serious rust issues but rather several recurring spots i'm getting tired of patching up every year so i'm (nearly!) committed to a full hull repaint job.

I'm seeking advice about the recommended number of primer coats below and above the waterline. It seems many folks agree on a minimum of 4-5 coats below the waterline but is there ground to those numbers? is it a matter of building a certain primer thickness? dependent on the primer used?

Similarly, at what frequency do steelboat owners blast and repaint their hulls? Or put differently, assuming a professional, quality job, how many years can I expect to get before having to worry about rust spots popping up?

thanks!

You don't want to go to all of that rouble and expense again, so don't spare the paint. Spray two-pack primer, 5 coats of two-pack epoxy, and 4-5 coats of two-pack Coppercoat. You won't be touching the hull again for 15 years then .

[s/v michaela]

Originally Posted by captlloyd

One of the finest steel boatbuilders East of the Mississippi told me to definitely not use zinc rich primer on my boat. Zinc rich primer is very weak, it will actually split in two, another words it will adhere to the steel and adhere to the finish coat but will actually split in between. I’ve seen it happen, big sheets of paint just stripping off in the wind. The boat builder was George Sutton.

I have one of his bigger ketches... so well built...actually overbuilt . He was a man who knew steel boats.

[Gypsyjon]

Originally Posted by Michael Cobbe

To stop rust use Zinc rich epoxy for the first coat.

If you can get is Zinc Chromate paint is the best. Blast the steel to white metal and get Zn Chromate on right behind the blaster while steel is still white. It will rust very fast after blasting while, We are talking minutes here. Blast some paint some, blast some paint some. etc

[Gypsyjon]

Originally Posted by s/v michaela

I have one of his bigger ketches... so well built...actually overbuilt . He was a man who knew steel boats.

If that happened it is because they did not get the zinc chromate on while the steel was still white blasted. They waited to long and iron oxide (rust) formed before they got the paint on . Timing is critical but easy enough. The paint guy just follows the blasting guy along. This is how steel tugs etc get painted.