Hot rolled pickled & oiled sheet - what paint system works?

PDW · Nov 10, 2022

Looking at the materials for building another steel boat. Last time I got hot rolled plate & had it professionally blasted & primed with a weld-through primer before cutting & welding.

Since then hrpo plate has become more easily available and in bigger sheets.

There's a significant cost savings going with the hrpo plate, but not if I can't get epoxy primers to stick without cleaning all the oil off first, unless it's really easy to clean.

So basically, how compatible is the plate with 2 part epoxy paints?

PDW

DDoug · Nov 11, 2022

A local shop uses a phospho cleaning (using a Hotsy).

Strostkovy · Nov 11, 2022

Our steel does not arrive very oily. If it does we wipe it with a blue paper towel before cutting. We just prep with a fine grit sandblast (120 aluminum oxide, I think), lightly torch off the fuzzies, and powder coat it.

PDW · Nov 12, 2022

Strostkovy said:
Our steel does not arrive very oily. If it does we wipe it with a blue paper towel before cutting. We just prep with a fine grit sandblast (120 aluminum oxide, I think), lightly torch off the fuzzies, and powder coat it.

OK thanks, it sounds like it's not the way to go for a boat hull. I suspected as much.

The point of p&o would be to avoid sand blasting. If it's going to need it, there's no point as blasting after the hull is built is a very unsatisfactory alternative - you never can get at all the bits where the frames and stringers are in contact with the hull plate.

I expect I'll just pay for all the plate, flat bar etc to be blasted & sprayed with a zinc rich weld-through primer. Adds a fair bit of cost but the epoxy paints like having the 'tooth' of a blasted finish anyway. Typically you're putting 400 microns of paint thickness over the steel to provide a decent barrier coating, it's not like powder coating at all.

You only get one good chance to paint a steel hull and that's when it's first built. The outside isn't so bad, it can be blasted back to white if necessary but - surprise - most steel boats rust from the inside because people skimp on the paint there, and leave mill scale, then once the interior is done, it's impossible to re-do effectively. My current boat is 4 years in salt water and the inside of the hull is perfect because I didn't want to do it twice.

PDW

gwelo62 · Nov 12, 2022

PDW said:
OK thanks, it sounds like it's not the way to go for a boat hull. I suspected as much.

The point of p&o would be to avoid sand blasting. If it's going to need it, there's no point as blasting after the hull is built is a very unsatisfactory alternative - you never can get at all the bits where the frames and stringers are in contact with the hull plate.

I expect I'll just pay for all the plate, flat bar etc to be blasted & sprayed with a zinc rich weld-through primer. Adds a fair bit of cost but the epoxy paints like having the 'tooth' of a blasted finish anyway. Typically you're putting 400 microns of paint thickness over the steel to provide a decent barrier coating, it's not like powder coating at all.

You only get one good chance to paint a steel hull and that's when it's first built. The outside isn't so bad, it can be blasted back to white if necessary but - surprise - most steel boats rust from the inside because people skimp on the paint there, and leave mill scale, then once the interior is done, it's impossible to re-do effectively. My current boat is 4 years in salt water and the inside of the hull is perfect because I didn't want to do it twice.

PDW

Interesting info.

Strostkovy · Nov 12, 2022

PDW said:
OK thanks, it sounds like it's not the way to go for a boat hull. I suspected as much.

The point of p&o would be to avoid sand blasting. If it's going to need it, there's no point as blasting after the hull is built is a very unsatisfactory alternative - you never can get at all the bits where the frames and stringers are in contact with the hull plate.

I expect I'll just pay for all the plate, flat bar etc to be blasted & sprayed with a zinc rich weld-through primer. Adds a fair bit of cost but the epoxy paints like having the 'tooth' of a blasted finish anyway. Typically you're putting 400 microns of paint thickness over the steel to provide a decent barrier coating, it's not like powder coating at all.

You only get one good chance to paint a steel hull and that's when it's first built. The outside isn't so bad, it can be blasted back to white if necessary but - surprise - most steel boats rust from the inside because people skimp on the paint there, and leave mill scale, then once the interior is done, it's impossible to re-do effectively. My current boat is 4 years in salt water and the inside of the hull is perfect because I didn't want to do it twice.

PDW

It doesn't need to be blasted. You could use an iron phosphate solution to prep it. But whatever material you use will need to be prepped somehow.

seiner · Nov 12, 2022

paint is not going to stick to unblasted steel. just a fact of life.

DDoug · Nov 13, 2022

seiner said:
paint is not going to stick to unblasted steel. just a fact of life.

What about hot rolled that has been dunked in acid to remove all the scale ?

PDW · Nov 13, 2022

DDoug said:
What about hot rolled that has been dunked in acid to remove all the scale ?

Yeah - that *was* the point of my original post...

Seems likely that the answer is either paint doesn't stick without further prep, or possibly some paints might but nobody here who's read the thread can definitively say.

So - not worth the risk IMO, better to buy hot rolled, have it pro blasted & primed, then start cutting & welding. Only need a light wash and treating the HAZ that way.

I've seen epoxy paint peel off of steel that was too smooth even though clean, not enough 'tooth' for the initial coats. Flap wheeling for example, 12 months in the water and the paint came off a new rudder when taking the anti-fouling back. We hit it with 40 grit on slow speed sanders and re-painted, that seemed to fix it. Friend's boat so I'd know if the paint came off again.

PDW

johansen · Nov 14, 2022

PDW said:
I've seen epoxy paint peel off of steel that was too smooth even though clean, not enough 'tooth' for the initial coats. Flap wheeling for example, 12 months in the water and the paint came off a new rudder when taking the anti-fouling back. We hit it with 40 grit on slow speed sanders and re-painted, that seemed to fix it. Friend's boat so I'd know if the paint came off again.

PDW

When I was researching the adhesion of epoxy to steel, I found some sources which explained that the limit is 1500 psi and the epoxy is adhering to the oxygen atoms bonded to the steel.. not the iron atoms.

The steel treatment they recommended was basically wipe it clean of oils, then 10% hcl then wash with distilled water, dry, then epoxy. Basically a controlled amount of rust.

Sand blasting or sanding the steel with 100 grit sand paper will increase substantially the surface area and thus the apparent adhesion strength. Diminishing returns.

Any good quality of paint will peal off the substrate if it's tensile strength is higher than adhesion. Of course if enough pigmemts are added then it would be too brittle and will crack. This will also give you a minimum thickness to calculate to ensure the coating cracks.

PDW · Nov 14, 2022

In fact I've found that the Jotun 605 epoxy primer (that the bastards have stopped making) stuck like shit to a blanket to lightly rusted steel. No further rust occurred but there was a really good bond.

I've got some sitting outside the workshop, in the weather, and no rust has reappeared in over 6 years now.

Naturally any paint won't do a damn thing for flaking rust and mill scale, all that has to go before painting. Hence the original query here.

PDW

seiner · Nov 14, 2022

in my experience with fishing boats, paint needs a rough surface to hold on to. The bonding to oxygen doesn't surprise me but the mechanical grip produced by a rough surface is Key. That the mechanical grip may only be a product of increased surface area also isn't surprising to anyone who has scarfed two pieces of wood together.

Freshly blasted surfaces are also generally very clean and that is also a requirement.
FYI I've used Devoe 235 bar-rust exclusively.

Now that I have Aluminum boats you might think that I treat thing differently. Nope. Clean, blast and paint with an epoxy barrier coat. I do take care to ensure that the blasting media is compatible with aluminum and will not cause electrolytic corrosion.

johfoster · Nov 15, 2022

seiner said:
paint is not going to stick to unblasted steel. just a fact of life.

Probably good advice for a saltwater boat owner, but certainly not a universal truth about paints and steel.

I don't think I would treat the HRPO any different than I would HR as far as prep. (my past uses for these materials are vastly different from yours, mostly architectural, and I am in a dry desert climate)

seiner · Nov 20, 2022

johfoster said:
Probably good advice for a saltwater boat owner, but certainly not a universal truth about paints and steel.

I don't think I would treat the HRPO any different than I would HR as far as prep. (my past uses for these materials are vastly different from yours, mostly architectural, and I am in a dry desert climate)

While am in a rain forest. we had 47" of rain in August and September. My brand new 14000gvw beaver tail equipment trailer was badly rusted in 4 years.

I am sure the trailer was just painted over factory produced steel surface. no prep at all.

604Pook · Nov 22, 2022

Have any local wheel abraders? For some jobs we will take hot rolled plate and have it wheel abraded prior to forming and fabrication. Nicer to work with and welding,, and saves on blasting it for prep for powder coating. Big bonus being all the inside lips that are formed are clean as well, which the blaster has a hard time doing.

Search

Largest Manufacturing Technology Community on the Web

Search

Hot rolled pickled & oiled sheet - what paint system works?

PDW

Diamond

DDoug

Diamond

Strostkovy

Stainless

PDW

Diamond

gwelo62

Aluminum

Strostkovy

Stainless

seiner

Aluminum

DDoug

Diamond

PDW

Diamond

johansen

Stainless

PDW

Diamond

seiner

Aluminum

johfoster

Cast Iron

seiner

Aluminum

604Pook

Aluminum

Similar threads