Which primer to use for inside of the hull for a steel boat?

[zeseb]

At the stern of my boat, below deck, I identified a few rusty surface, nothing major but for the sake of maintenance, I sand it to remove most of the rust, apply some rust converter with a rag, then I apply some primer.

12 months later, the primer had disappeared for rust again... see picture

I am not sure I am sure I am doing a great job because I think I don’t know how to do it.
On the surface I can see some sort of drops of rust emerging randomly.

Do you have any advice how to deal with it?

[mickt243]

Primer isn’t an anti corrosion coating as such - sure, some do have anti corrosion properties , but it’s main purpose is to make paints or other coatings stick to a surface it wouldn’t ordinarily stick to.

I’d use a primer that’s zinc heavy, then paint with several layers of something like ‘killrust’.

Also, make sure that after you use the rust converter, many of them need to be washed off or they cause accelerated corrosion....

[barnakiel]

Imho.


Needlehammer and then ... epoxy epoxy epoxy epoxy ....


There is a thing called zinc loaded epoxy. It is said to be very good on internal hulls too. Google your friend.


b.

[Delfin]

Get some Ospho, that converts the rust to a stable substrate. Then anti-corrosion primer, then two part LP primer, then top coat. AwlGrip or Ameron have complete systems that will do the job.

[hpeer]

Clean as good as you can.
Rust treat.
Wash to remove acid.

Plan A - Best
One coat Ameron Amercoat 302 zinc rich primer
Two coats Ameron 235
“Hot coat”, that is put next coat on while first coat is still deformable to your fingernail.

Plan B - better
Two coats Ameron 235

Plan C - near as good as B
Two coats of POR-15 (Paint Over Rust)
Then POR recommended top coat

Amercoat comes in gallon cans with a separate can of activator. It is a epoxy that must be mixed. For small batches it is a real PITA. The zinc epoxy is about 90% zinc powder, nearly impossible to stir up, it can be done. Using the epoxy methods are best if you are doing decent size areas. For small touch up spots it’s hard to justify.

The POR15 can be had in small containers, they will sell a 6-pack or 4 Oz containers. This makes it very convenient for small touch up spots. But it needs to be top coated. Although inside with no UV that may be optional.

https://www.por15.com/

[RaymondR]

You are always going to have problems in the rusty spot in your image because there are places between the frames and the plating where you cannot get the corrosion out of or paint into. The rust in the crack will corrode and lift under any paint you put there and progress away from the frames slowly but inevitably.

I gave up on epoxy paints internally after trying many in the same situation you have and started using just a couple of coats of Marine Penetrol followed by a slurry of white cement mixed with boiled linseed oil. It works just as, if not better than, epoxies. The basic nature of the cement seems to slow down the corrosion and the white colour and absorbent nature of it when it dries shows up any rusting pretty well as soon as it happens.

[sy_gilana]

In addition to the above, the best conversion primer I have found is BRUNOX.

[Delfin]

Originally Posted by sy_gilana

In addition to the above, the best conversion primer I have found is BRUNOX.

Have you used it on aluminum?

[Lemsteraak]

I had heavy corrosion in the bilge of my boat. The best is just to cut out the area and replace with new steel. Never had good luck with epoxies or zinc rich primers, it was just painting over rust.

The best bilge coating I found was what the Dutch use on their inland cargo barges. Sort of a waxy black oily coating that takes about a month to dry. Gets into the pores and keeps the oxygen away. You buy it in 5 liter containers, dirt cheap.

Toplicht has this which sounds like the same stuff

https://www.toplicht.de/en/shop/yach...oel-hansalux-u

[RaymondR]

Originally Posted by Lemsteraak

I had heavy corrosion in the bilge of my boat. The best is just to cut out the area and replace with new steel. Never had good luck with epoxies or zinc rich primers, it was just painting over rust.

The best bilge coating I found was what the Dutch use on their inland cargo barges. Sort of a waxy black oily coating that takes about a month to dry. Gets into the pores and keeps the oxygen away. You buy it in 5 liter containers, dirt cheap.

Toplicht has this which sounds like the same stuff

https://www.toplicht.de/en/shop/yach...oel-hansalux-u

This stuff sounds like wax oil. I made some by heating a few large candles in thick gear oil. Used it in difficult to access places.

One of the other things I used in the V where the hull plasting joined the stem bar was pitch. I poured hot pitch into the top end after warming the steel. When I removed some about 25 years later I found that the hot pitch had "wetted" the steel, hence good adhesion and sealing, and there was no corrosion.

[zeseb]

Originally Posted by RaymondR

You are always going to have problems in the rusty spot in your image because there are places between the frames and the plating where you cannot get the corrosion out of or paint into. The rust in the crack will corrode and lift under any paint you put there and progress away from the frames slowly but inevitably.

I gave up on epoxy paints internally after trying many in the same situation you have and started using just a couple of coats of Marine Penetrol followed by a slurry of white cement mixed with boiled linseed oil. It works just as, if not better than, epoxies. The basic nature of the cement seems to slow down the corrosion and the white colour and absorbent nature of it when it dries shows up any rusting pretty well as soon as it happens.

Hi RaymondR, before applying the Marine Penetrol, do you treat the rust before (Rust converter like Ospho)?
Thanks,

[RaymondR]

Originally Posted by zeseb

Hi RaymondR, before applying the Marine Penetrol, do you treat the rust before (Rust converter like Ospho)?
Thanks,

I never used rust converter anywhere on the boat.

My usual practice was to use a rust remover called CoroDip which mixes 5:1 with water poured into the bilge and circulated around using a small 12V submersible pump. I would move the tube around and assist removal of the rust with a wire brush.

For big jobs I would use a needle gun and an air compressor.

[hpeer]

I’m reminded of another paint.

Petit make a ureathane one part primer called RustLoc. It has a very high aluminum dust content. Prep is like all others, it requires at least 3 coats, I sometimes use as many as 7,applied before the previous coat is completely dry. It provides a very hard coating that works well. It is very runny which can be good or bad. It comes in as small a quart cans.

The disadvantage is that if you don’t get the can lid COMPLETELY clean or the paint the lid will permanently attac to the can. We transfer it to Ball jars and use somw aluminum foil to keep the threads clean. Then if the lid gets welded on anyway I just cut through the lid with a knife to get at the paint and transfer the balance to a new ball jar.

[SollaSollew]

Originally Posted by hpeer

Clean as good as you can.
Rust treat.
Wash to remove acid.

Plan A - Best
One coat Ameron Amercoat 302 zinc rich primer
Two coats Ameron 235
“Hot coat”, that is put next coat on while first coat is still deformable to your fingernail.

Plan B - better
Two coats Ameron 235

Plan C - near as good as B
Two coats of POR-15 (Paint Over Rust)
Then POR recommended top coat

Amercoat comes in gallon cans with a separate can of activator. It is a epoxy that must be mixed. For small batches it is a real PITA. The zinc epoxy is about 90% zinc powder, nearly impossible to stir up, it can be done. Using the epoxy methods are best if you are doing decent size areas. For small touch up spots it’s hard to justify.

The POR15 can be had in small containers, they will sell a 6-pack or 4 Oz containers. This makes it very convenient for small touch up spots. But it needs to be top coated. Although inside with no UV that may be optional.

https://www.por15.com/

Hey hpeer,
I'm having an issue with osmotic blistering through several coats of 235 put on freshly sandblasted clean steel. I'm now starting to wonder if it's because of our not doing a primer coat of 302 (btw, the yard didn't recommend it). Your thoughts?


(My specs: Amazon 37, steel hull. Blisters have water in them, go through all coats down to shiny no-rust metal. Have tried application in 2 different climates.)


Thanks.

[hpeer]

On using 302 as a primer I’ve heard all the arguments and they are about a wash. The bottom line is for inside it’s fine, for outside the “experts” (including Ameron representatives, are split. I have and do use it.

As to the blisters I sandblasted my hull and got those blisters in spades BEFORE launching. Some were full of water, some not. EVERY time I got the problem was in DELAWARE City, DELAWARE near a refinery. It did not happen everywhere on the hull but at times it DID occur on 100% of the repainted area. It has NEVER happened in Newfoundland.

The one thing all the Ameron reps agreed upon was that I was getting acids on the hull under the paint. In Newfoundland the air is clean, no industry, no problem.

Given my experience I would look to something contaminating the air and thus the metal. I DID use 302 primer, same issue so I don’t think it’s that.

I’m sorry to hear of your problem, very frustrating.

In the post you quoted I omitted one solution. Pettit paints RustLoc. I’ve been using it more and more with fairly good results. It would not be my first choice for a bottom job but has its applications.