Products to fill in rust pits

[KelseyB]

Hey everyone-

So I have a steel boat, and she has an icebreaker-esc long keel with a flat bottom and a decent amount of width (over a foot wide on the leading end, narrowing to a point at the aft end). I did a refit a couple of years ago, and one of the things I tried to address was some moderate to severe pitting on that bottom edge of the keel- aggressive wire brush, ospho, amercoat 235, then bottom paint. After getting hauled again after about a year, that method didn't hold up. The pits aren't any worse than they were before, but the amercoat didn't hold so they're exposed again. We're looking for something more substantive to fill in and protect the pits (after they've been cleaned out again) that will adhere to the metal and that paint will adhere to overtop of it. It's a decent amount of surface area (a couple square feet), so that plays into it, as well.

I've heard some good things about Corroseal, anyone have any experience with it? Other than that, we're thinking of some sort of 4800 of 5200-esc product, that could be painted or caulked on, that has decent thickness to even out the texture a bit, and will adhere well to the metal below the waterline and not shed off paint... does this product exist? Would 4800 or 5200 be effective, albeit expensive?

[MicHughV]

Your best bet is to have the rusted areas sand blasted. It's a pain in the butt, but if you are really concerned that is the only way to go.

Some steel plate is likely already pitted from laying in the supply yard for a very long time. When I built my steel boat, I found many pieces with pitting, but it didn't affect anything.

I had a steel boat. The bottom keel plate was 1/2" thick...approx. 12 mm. I would think that most steel boats have a similar thick plate. Some rust should not be an issue.

It is easy to determine. You buy an ultra-sonic steel thickness gauge online. Many varieties exist. Around $200 and up and you can get a pretty good one.

Steel can throw off a significant amount of rust before it deteriorates.

The usual paint for steel after sand blasting is some type of zinc chromate paint. Many varieties exist. The paint is quite thick. Several coats will likely fill any pitting.

[KelseyB]

Originally Posted by MicHughV

Your best bet is to have the rusted areas sand blasted. It's a pain in the butt, but if you are really concerned that is the only way to go.

The usual paint for steel after sand blasting is some type of zinc chromate paint. Many varieties exist. The paint is quite thick. Several coats will likely fill any pitting.

Yea in the long run I definitely agree with the sandblasting, but unfortunately at the moment we don't have access to that so we're looking for something that will tide over for a year or a few.

Would a zinc chromate paint be alright even if not sandblasted (just aggressively cleaned out)? Any brand recommendations?

[KelseyB]

What about Pettit Rustlok primer? Anyone have any experience with it? It looks like it might not necessarily fill in the pits, but I'm not looking for race-ready smoothness, mostly just something that will adhere and protect, any smoothing would really be a bonus. Thoughts?

[MicHughV]

I used to chase after rust spots on my boat.

I would use a dremel tool with a rounded metal file bit, about the size of a match head. I could poke this into the very small crevices I found, plus clean the surrounding area. After that I would coat the area with OSPHO. I would then paint it with a zinc chromate paint and finish with an epoxy paint. You should be able to get ZC in a small can or spray. The little Dremel bit wears out quick, and you may have to get several of these.
There are several products on the market similar to this, depending on your location.
Check Amazon and you will find dozens of products, both in spray cans and smaller cans.
You can also check Home Depot. They also have a variety of stuff.

I never had problems with my boat below the water line though, but I'd likely use the same method, ie, clean with dremel tool, OSPHO or other primer......and then the bottom paint.

Finally, I did have one spot that required some major work. At a loss, I used WEST system epoxy with some filler. Amazingly, it worked like a charm and I never had a problem with that spot again. While I was building my boat...just for funsies, I painted a small piece of flat bar with WEST and hung it in the water...salt water....I would check this bar from time to time, but after several years it was still as good as new. The bar was just rough cleaned with a wire brush before it went into the drink.

[KelseyB]

Originally Posted by MicHughV

I used to chase after rust spots on my boat.

I would use a dremel tool with a rounded metal file bit, about the size of a match head. I could poke this into the very small crevices I found, plus clean the surrounding area. After that I would coat the area with OSPHO. I would then paint it with a zinc chromate paint and finish with an epoxy paint. You should be able to get ZC in a small can or spray. The little Dremel bit wears out quick, and you may have to get several of these.
There are several products on the market similar to this, depending on your location.
Check Amazon and you will find dozens of products, both in spray cans and smaller cans.
You can also check Home Depot. They also have a variety of stuff.

I never had problems with my boat below the water line though, but I'd likely use the same method, ie, clean with dremel tool, OSPHO or other primer......and then the bottom paint.

Finally, I did have one spot that required some major work. At a loss, I used WEST system epoxy with some filler. Amazingly, it worked like a charm and I never had a problem with that spot again. While I was building my boat...just for funsies, I painted a small piece of flat bar with WEST and hung it in the water...salt water....I would check this bar from time to time, but after several years it was still as good as new. The bar was just rough cleaned with a wire brush before it went into the drink.

The first suggestion is what I did last time, minus the zinc chromate, so I'll definitely look into that because I already have everything else, which is helpful.

And I'm surprised the west system worked- I thought of that but I was worried it wouldn't hold up over time, do you think it would be as effective on a larger area and on an active boat, rather than just the bar? I worry about it having trouble adhering straight to the steel

[GreenWave]

Originally Posted by KelseyB

The first suggestion is what I did last time, minus the zinc chromate, so I'll definitely look into that because I already have everything else, which is helpful.

And I'm surprised the west system worked- I thought of that but I was worried it wouldn't hold up over time, do you think it would be as effective on a larger area and on an active boat, rather than just the bar? I worry about it having trouble adhering straight to the steel

My suggestion-
1) Wire brush the area
2) Apply Corroseal or equivalent per instructions from manufacturer
3) The apply some sort of zinc primer or alternative
4) then fill and paint as desired.


Corroseal works, but it doesn't fill in pits.

[MicHughV]

I sand blasted my boat inside and out.

After blasting the inside, I coated it with a two part coal-tar epoxy.
Some 3-4 weeks later, I discovered I had missed painting a section underneath one of the cockpit seats.
Having run out of paint by this time, I wire brushed the surface rust off and applied a coat of clear WEST.
Never ever had a rust problem with that area in the 15 odd years I owned that boat. Mind you I had sand blasted that section a few weeks earlier, and although it had rusted some, it was basically clean.

The sand blasting leaves an " etching" in the metal, which makes for a good bonding surface to paint, and also good for the WEST.

Based on my own experience, I would not hesitate to repair some keel pitting with it.

Here's another thought. Some years ago, I backed my truck into a dumpster which dinged the metal bumper pretty bad. After a while the dinged bumper started to rust pretty bad.
I bought some body shop filler material, which you can get at any auto store and applied this to the bumper. It has tenacious gripping qualities. Using this filler material I built up the ding and painted it white , as the whole bumper was painted white.
This was some years ago, and it looks as good now as it did when I did the patch.
This could be an excellent fill material for your pitting. It's a two-part material, a base with a hardener. You have to mix it in small quantities as it sets up quickly.
Like the WEST, I would not hesitate to use this product to fill pitting in the keel. Slap a coat of paint over it, and it is good to go imo.

[ShoreFun]

First off I will state I have no experience with steel boats.

I do have some experimental experience from doing car restoration. In particular I have restored a very rusty and very rare car. As part of my work I experimented as I have been told many things. I also have contact manufacturers on how products work.

First off, rust pits. Unless you get rid of the rust spot at the bottom of the pit it will just come back. There is oxygen and moisture trapped in that brown stuff. You can try all sorts of acid products thinking you will cure it, you wont. I will qualify the issue is you need enough acid in contact to fully convert the iron oxides. That does not happen. I can assure you the rust will blossom from the bottom of the pit. Yes, I tried various products on a rusty part that I did not blast the rust from the bottom of the pits. Yes, the rust came back stored inside.

I have removed paint from variously prepped and painted parts I have bought. In each case you could see rust coming from the pits radiating out. No, the paint was not well adhered at that point.

So let up talk epoxy. I was told by the owner of a paint company, Southern Polyurethanes, that epoxy needs a base to cure. If you use an acid product on the metal and the surface still is slightly acid then the epoxy can't cure at the surface. That means the epoxy is not adhered to the surface.

Zinc Chromate is not intended for any sort of rust control. At least in my talk with Dupont. Paint needs a 'rough' surface. That can be made either by mechanical processes such as sanding or sandblasting or chemical. For new metal you can use zinc chromate to form a bond. The problem is most people do not follow the instructions. I have seem MANY 'restored' cars where the paint is chipped showing nice smooth zinc chromate paint still there. Which means the next layers of paint failed to adhere to the Zinc Chromate.

The is a product called POR15 and some copies out there. Their claim to fame is it will stick to rust. I have used it and it sticks to everything including skin. The problem is rust is not well stuck to the metal. Plus it does not always stick to clean metal so well. I have personally done the prep and later seen the paint come off like a nicely molded part.

Poorly bonded products can allow water to get in behind them. I have seem POR 15 and epoxy surfaces get moisture behind them and severe rust occured that could not be seen till way too late.

Hopefully the above info can help you make better decisions. I have no idea on how to best apply it to the marine world.

My best advice is to follow manufactures directions and not Bob who has 50 years experience. Pretty sure the chemist knows how the product works better then Bob.

[MicHughV]

Well, I built my own steel boat, and it was my home for about 15 years. I also got to know several other people with steel boats and rust and prevention thereof was a popular topic of conversation. My boat is now approaching 40 years of age and on it's third owner and still rust free and doing good.

The methods I described above worked fine for me, but I was diligent in removing any visible rust. Off course, sand blasting would be my no.1 choice, but this was not always available to me, so had to devise other ways with tools and materials at hand.

[KelseyB]

We've just stumbled across Jenolite- has anyone ever used Jenolite rust remover? Thoughts?

[flyingnut40]

I was going to suggest POR-15 as an option as they claim that is tolerated a light rust (read flash rust) after sand blasting...... and they claim as a MCU (moisture cured urethane) it gets stronger with exposure to water.
I emailed the company to see if that "moisture" included salt water and if it would work well with a submerged steel hull and the response I got back was the 3 step procedure for post sand blasted steel. Boiler plate stuff..... Not specific OR very helpful. Makes me want to reach out to Sherwin Williams about their less expensive but less aggressively marketed MCU's

[nortonscove]

I've used POR-15 on my cast iron keel to do touchups and it works well enough. I don't expect anything to keep rust at bay forever but a well prepared surface has meant not having to rework the areas in more than half a dozen years.

[skipmac]

Originally Posted by KelseyB

I've heard some good things about Corroseal, anyone have any experience with it? Other than that, we're thinking of some sort of 4800 of 5200-esc product, that could be painted or caulked on, that has decent thickness to even out the texture a bit, and will adhere well to the metal below the waterline and not shed off paint... does this product exist? Would 4800 or 5200 be effective, albeit expensive?

Used it, love it. At least in my application it worked better than anything else I tried.

PO of my boat had a drip from the anti-syphon fitting right onto the bell housing and transmission on the engine. Left it for years.

The rust on the transmission was so bad that I peeled large 1/8"-1/4" thick flakes off parts of the case. I tried wire brush, multiple solvent cleaning, then primer and paint a couple of times. Lasted a few months before starting to fail.

A few years ago painted the area with Corroseal and not a spec of rust has come back.

Don't know if it would work as well below the waterline but worth a try. As I recall, you do NOT clean all the rust off. Corroseal works chemically with the rust to form the barrier. Knock off the loss stuff, flakes, etc but don't get down to clean, shiny bare metal. I never got around to painting mine but of course on the bottom coat it well.

[Ninedotsix]

I can appreciate the problem.
Using Ospho as you’ve done helps to “burn” out the rust, and then a product that performs like P-O-R will base coat, well.
Check compatibility of your bottom paint with the P-O-R, as that’s really important.
WaterTite would work as a cosmetic filler, but might want epoxy resin coat or two over the P-O-R before WaterTite.
Don’t bother with zinc chromate or other zinc-rich paints, as the salt water will very quickly remove the Zn. Good in air, not so much submerged.
I’m double checking myself on application of P-O-R, and will post a correction if indicated.
~ 9.6