rusty steel hulls

i would invest in a couple of good needle guns and a lot of labor

Welcome to YF Stiletto28. It all depends how much area you need to work and its accessability. I also assume you done or plan to do a radiogauge to check the plate thickness. Search the files here. Much has been discussed.

Here's one link:
http://www.yachtforums.com/forums/steel-hull-issues/2985-hull-questions.html?highlight=removing+rust

If the rust isn't to bad Tavler is right, invest in a needle gun and a wire brush. Clean the areas as best you can and follow with Ospho or something like Corroseal which converts the rust and seals the metal from further corrosion. This can be followed up with an epoxy paint system. Years ago I did this on a Army Corps Engineers boat in a machinery space and I saw the boat several years later and it still looked good.

Interior coating

You might want to take a look at C . I. M. (Chevrons Industrial Membrane) a liquid rubber membrane for the interior. Properly applied, it's there forever.

Stiletto, immediately after sandblasting/needle gun/whatever, down to what they call "white metal"....add a coat or two of zinc primer.

Before doing anything of the sort I would determine what coating system I want to use and follow the procedures prescribed by the manufacturer of that coating.

Adding a layer of something that may or may not be compatible just leads to a duplication of work and expense, or a failure of the coating system in the near future.

steel hull

For forty years I (my company) sandblasted and painted steel water tanks. We came across the Chevron product many years ago with a great degree of success, again give proper surface preparation and application. see
www.cimindustries.com

a lot of good advice here as an owner of a steel boat for the last 40 years are so i would evaluate wich coating system i was going to use (as marmot sujested) and go for there we have used the ppg amerlock system for a few years now and have had good luck , there are other systems that are quite good i would evaluate wich would be best for your need,s

just a thought travler

Marmot, unlike many here who 'shoot from the lip', I have personal experience in building and maintaining A36 steel-plated expedition vessels. As advised by Joe Purtell of U.S. Paints/Azko Nobel, inorganic zinc primer on white metal finish is/was the default and initial coating in the process, followed by brand name (Interlux or Awlgrip) epoxy primers, then layers of fairing material and finishing with a brand name linear polyurethane.

At least that's how it was done, per the experts. Did some other coating--incompatible with zinc-- come on-market in the past couple of years that puts egg on my face?

I am by no means an expert and would hope that one weighs in on this.

Not that I know of. It is just good practice to select a coating system and prepare in accordance with that manufacturer's instructions before using any third party product that will underly that system.

No offense intended, it is just that there are many "zinc primers" on the market. Some of which may or may not use a carrier or solvent compatible with all coating systems.

Given the cost of hull preparation and application of a marine coating system it is simply a prudent approach to carefully plan such a project.

Inside, especially in an engineroom, I prefer CO2 blasting.

In another life I painted a lot of steel boats and the zinc primers were compatible with any epoxy topcoat. But I agree with Marmot it is unwise to mix different manufacturers products it's set up for failure or beter yet if there is a warranty issue your out of luck.

CO2 blasting uses crushed dry ice for media. It has its place but not too sure if it is the best thing to use in the hull interior. It is expensive, noisy, and requires extra ventilation so as not to asphixiate everyone in the interior. The rust and paint chips still need to be cleaned up so the work is no less. The only place I have found it to be really useful is in an area where you simply can't afford to have any blasting media residue, but don't have access to the areas behind or under the work, such as removing hard packed soot from the fin tubes of a boiler economizer. But I don't think you have one of those on your boat.

There are vacuum nozzles which reduce the junk to almost nil so cleanup and "overspray" are no longer an issue, much less an "epic mess." If the thought of highly abrasive grit bothers you, there is soda blasting. It uses plain old ordinary baking soda that is about as "green" as it gets and won't come back to haunt you in the future. Combine that with a vacuum nozzle and life is good.

Surface Prep

Dry ice is a slow process but very effective. You can also use High pressure liquid blasting. Generally runs 40,000 psi water pressure to blast clean any rust there. However you don't get the original Mil profile that was once there and may need mechanical or abrasive blasting to achieve a good profile for your primer due to rust eating away the profile. As far as a primer on A36,56, or 105 steels. I kinda move away from Zinc primers. Due to galvanic action the zinc erodes away protecting the carbon steel leaving voids,blisters between the primer and top coat. This is why you have zinc bars through out the hull. If you have cathodic protection incorporated into you electrical system it works fairly well in this application however. If you use a organic or inorganic zinc use an epoxy equal to it that will allow even erosion and wear so you can tell when a problem appears. Zinc paints usually need a system to keep the solids stirred while painting for even application as well. An easier but effective way is what I typically use on most steel hulls. I generally use a Sherwin Williams product Macropoxy 920 primer high solids over the prepared area. Then top coat with Macropoxy 646 fast cure epoxy below deck areas. If outside I will top coat with Acrolon 218 two part polyurethane for UV protection above the waterline. As long as you keep the carbon steel protected from oxygen molecules (oxidation) it usually does fine. Devoe,PPG,Sherwin,Americoat, and other paint suppliers will give you a free survey on what you will need for different applications. Also you can look up NACE or SSPC on the web and get great info on marine coatings. I have been in marine coatings for 20 years and key advice is get someone to look at it. No two vessels are the same. Send me your e-mail and I can give you some contacts in your area. Hope this helps.

There is also a process that uses CO2 pellets and CO2 in it's supercritical state. Supercritical CO2 is a excellant solvent . IIRC the critical pressure is 1068 psi for scCO2.

On a side note;
You can google to read about scCO2 since you'll be hearing a lot more about it as it is finding it's way into HVAC systems and autos and eventually yachts.

scCO2 in a supercritical ,transcritical system can carry 2.5 time the heat than standard CO2 refrigeration systems. The main engineering problem is the high side is ~2000 psi and the low side ~500 psi requiring a strong compressor and evaporator with micro channels. Seems current evaporators,at least on autos leak at 50 psi.

Ceramic Bead Insulation Paint

Hey Loren, ever hear of this?

I am totally against spray on PU foam since it makes a mess of the hull and cannot weld or cut on the hull when fitting out. Furthermore, should condensation forms behind the foam where adhesive to the hull was not complete, it is hid from view and a disaster in the making. Loose PU sheets fitted to hull and deck behind paneling makes more sense. Polystyrene has no place on a boat for a many obvious reasons.

However, my last steel boat built 2008/9 I used ceramic bead insulation paint (spinoff from NASA space program) on the inside of hull and deck. Thickly applied by brush and roller and having used white color, the hull is "neat" inside. Condensation does not bother this coating and should condensation formed inside the hull still it is able to drain to the bilge sump through its drain holes in stringers etc. Magic stuff and one can still cut and weld when doing fitting out if needed and just a matter of touching up the paintwork and applied fresh ceramic stuff over area.

Best part, it does not burn and cost less than spray-on PU foam
...more here
Steel hull isolation: alternatives to foam? - Page 10 - Boat Design Forums