Products to fill in rust pits

[Ninedotsix]

Looks like you’ve got lots of input, and I’ll rest with my previous post, no correction provided.
Cheers

[thinwater]

(I do inspections of steel oil tanks professionally--API certified inspector)


For certain, get a UT meter. They've come way down in price. It won't be pro-quality, but it will work as long as you calibrate against a piece of known plate.


Get a pit gauge. Less than $20 on Ebay. It is impossible to read pits by UT (too rough) and impossible to gauge depth by eye.
As for repairs, lot of good advice on coatings. But you should also be prepared for welded repairs. It's not that big a deal. If the pits are small and deep (1/3 plate thickness, perhaps), say finger tip in area or less, just fill them up and grind level (pit welding). If it's a larger area, more than a few square inches, get a certified welder to replace plate sections. Good repairs can be good as new. The plate should be exactly the same thickness as original.



Patches welded on are generally an abomination. Water will get between the layers, rust, and they are impossible to inspect properly.


Patches should have rounded corners (there are codes) in most cases. A square corner creates considerable stress and often causes cracks. There are exceptions when the patch butts up to something square. There are also rules about avoiding welding near existing seams. A code welder will know all of this.

[Ninedotsix]

Originally Posted by thinwater

(I do inspections of steel oil tanks professionally--API certified inspector)


For certain, get a UT meter. They've come way down in price. It won't be pro-quality, but it will work as long as you calibrate against a piece of known plate.


Get a pit gauge. Less than $20 on Ebay. It is impossible to read pits by UT (too rough) and impossible to gauge depth by eye.
As for repairs, lot of good advice on coatings. But you should also be prepared for welded repairs. It's not that big a deal. If the pits are small and deep (1/3 plate thickness, perhaps), say finger tip in area or less, just fill them up and grind level (pit welding). If it's a larger area, more than a few square inches, get a certified welder to replace plate sections. Good repairs can be good as new. The plate should be exactly the same thickness as original.



Patches welded on are generally an abomination. Water will get between the layers, rust, and they are impossible to inspect properly.



Patches should have rounded corners (there are codes) in most cases. A square corner creates considerable stress and often causes cracks. There are exceptions when the patch butts up to something square. There are also rules about avoiding welding near existing seams. A code welder will know all of this.

Great resource you are, thinwater!!
Surely an abundance of knowledge and experience.
I was thinking along about plate repairs as you were posting your reply, as I found myself wondering how thick the plate was originally, how much is left, and since the area of concern is on the forward end of the keel, how well it would stand up to a hard grounding? If the pitting is pervasive over an area, seems like it could go Swiss cheese at some point.
That said, I did look at the PPG Amercoat 235, see it is an epoxy, and now retract what I wrote earlier about POR, as that particular product as was pointed out is MCU, seems better suited to topsides. Likewise Corroseal, which is latex based.
What does thinwater think of using coal tar epoxy?
~ 9.6

[rmlarson1098]

Originally Posted by 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?

Weld it. Or more accurately, 'pad weld' it.
Pad welding is not an advanced technique. If you can weld at all, you should be able to accomplish it with a few hours of instruction/practice.
A simple wire feed welder should work. 'Cored wire (like Innershield) that doesn't require an external shielding gas supply. Get a piece of plate steel (scrap) about the thickness of your hull/keel, practice running consistent, tight beads.
Keep the arc short. Get a number 9 dark lense for your hood if you're working outdoors. If you don't have a welding hood (helmet) just get a 'old school' hood. Don't spend a lot of money on a 'blinker' aka as an electronic hood that darkens automatically.
Either sandblast or grind the weld area on the boat first, of course. Keep in mind that overhead welding (should it come to that) is not much different than flat welding - just upside down.
Don't let that intimidate you.
Any vertical welds should be downhand. Don't run vertical up.
If you get porosity in the weld, make it stop. Lay the beads tightly, over-lapping maybe 50%. Slow down! Take your time. Most novice welders weld way too fast. Don't leave grooves (aka 'wagontracks') between the beads.
When you have plenty of nice, tight weld beads built up, grind it smooth but not quite flush. Just fair it nicely and leave a 1/16" of build up or so.
Pad welding - building up the hull with weld - is a relatively simple skill to learn, and very handy for steel boat owners.
Of course if there is anything inside the keel/hull that is combustible, switch to plan B.
Best of luck

[mvmojo]

Originally Posted by KelseyB

Hey everyone-

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?

I've used Corroseal extensively and it has worked great on MOJO. Use a dremel to grind out the pit so that there is no loose rust. It does not need to be bright shiny steel. In fact, it's actually better if there is tight rust. Corroseal reacts with rust to convert it to a compound called magnetite which will not oxidize (rust). 24 hrs (or more but no longer than 30 days) after applying Corroseal, fill the pit with West System epoxy thickened with West System 404 high-density filler. It's import to fill and fair the pit otherwise micro turbulence will eventually "pull" the paint out of the pit (as you've experienced). Once the West System has thoroughly cured, paint the bottom as usual. No need to top coat the West System first, just paint with the bottom paint of your choice. We used Ameron ABC and routinely got 4+ years out of it.

[Boatyarddog]

Originally Posted by KelseyB

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?

I used a paintable rust converter after sanding as much rust as I could off.
Then bottom paint.
Boatyarddog

[Oceanride007]

Lots of product names mentioned above. Bit confusing, but it should be remembered that corrosion will occur through any paint system that does not have sufficient insulation, water resistance and bonding strength. Hands down Epoxy is the way to go.
I have also had the problem with no access to sand blasting, and therefore my results are less than 100%, Have tried rust converter, but my go to solution is a huge variety of wire brushes and burrs, sometimes to get rid of a weld spatter I'll use a angle iron. Next is severe cleaning Oxalic acid (Sugar soap over a broader area) then use a skinny epoxy (I use carboline Altralock 576) this claims to mechanically lock the corrosion particles at the molecular level takes a while to dry. If using Epoxy paint you need 3 coats. Sometimes I use neat West system 5:1, I like a smooth surface so that if corrosion occurs again, you can see it, and next time be more aggressive in your cleaning. Epoxy needs like a finishing paint, think best to apply when previous coat can still get your finger nail in, don't wait too long to get your protection system on, I've been able to complete a repair in a day.. I prefer small plastic shot glasses and ice pop sticks I go through lots, and I make accurate mixes.