Insulating while refitting rear cabin

Hi.

I’m working on refitting our rear cabin and have removed a lot of rust and painted with good epoxy paint.

Now I’m getting ready to insulate the ceiling and sides with 19mm Kaiflex, and have read it is important to insulate both plates and frames to prevent condensation on them.

But how do I attach the wood to the hull to be able to have something to screw the walls and ceiling on to ?

 

You can attach the ceiling directly to the stringers and beams with self tapping screws.

 

So I should cover the stringers and beams with thin foam, I have some 3mm thick 50mm wide strips, then use the 19mm between them and screw some wood battens through the foam and into the frames ?

 

Welcome to the Forum Doc!

Your re-fit work in the photos above is looking very impressive.

The attached article by Danny Greene does not directly answer your answer re insulation - he has a plywood liner with an air gap between the plywood and the steel, and his deck / cabin top is timber - but he presents a good case for having all of your inside furniture arranged as removable modules.
Will you be staying in high latitudes with your steel boat, or heading south at some stage?

 

That seems like a significant loss of section on the plating and framing. Before continuing, you should measure how much loss there was, and how deep the pitting is.

 

Thanks, it has been a lot of work to get this far, and now I want to make sure the insulation is done in the best way possible.
We will most likely stay up north, but have talked about going down the canals, so the boat can visit its birth place The Netherlands

I’ll take a look at the article, and have planned for easy access to both walls and floor.

 

Yeah, I was a bit worried at first, until I read that rust is 7-10 thicker than steel, and I actually bought an ultrasonic gauge to put my mind at ease.

The boat is made of 5mm steel all over and the worst pitting is down to 4mm, but it’s mostly between 4.2mm and 4.6mm,

I also hit it pretty hard with a hammer and a big screwdriver without making a dent, so it seems the plates has got som life left.
The keel is mostly inaccessible so I’ll take some measurements and keep an eye on it.

it seems the corrosion was much worse on the starboard side and aft in the cabin, maybe because of the heat duct and the steel ballast being located there.

 

Sounds good. You did a thorough inspection.

 

loo

Project is looking good!

Hull paneling can be screwed to plywood strips attached to frames or stringers. (Ply works better than solid wood here, less cracking). My boat has small stainless tabs tacked onto the stringers, Then the ply strips bolted to that. My paneling screws onto the ply strips.

Many choices for insulation. I appreciate the idea of removable foam to inspect or repair, but nothing works for the intended task like two part polyurethane spray foam. Water will not condensate under that. Ultimate job is to coat the foam with a rubberized paint like bedliner, (thin coat).

 

I have actually thought about welding something like small steel angles to either the stringers or the hull to hold the wooden frames. But then again maybe I’m over engineering it, and could just use the same method as original, where small plywood plates where pressed in between the hull and stringers to attach the wooden frames to.

Today was a total disaster day, as I wanted to paint some white polyurethane paint on the bottom of the cabin, but the paint developed pinholes all over, and I had to rinse everything off again.

I think the problem was that the hull was too close to the dew point, or maybe the epoxy primer wasn’t fully cured. Never the less 4 hours of work, a can of paint and two cans of thinner, and I got nowhere.

Next time I’ll pay more attention to the dew point, that’s for sure

 

That does look really strange. Isolated pinholes (holidays) are usually from surface contamination like oils and waxes. I have never seen a two-pack polyurethane pinhole so much. I would double check: the correct reducers and percentage are used, catalyst mixing ratio, and air filters on your compressor / gun. A bit of oil or water in the air supply could definitely cause that.

Side note- why do you wish to apply LPU over primers that will be covered by paneling? We typically just build up with epoxy primers until desired mils (thickness) is achieved and insulate. The epoxy primers will last for decades without direct UV exposure.

I would proceed with stripping back the LPU, careful solvent cleaning with proper all cotton rags- change often, and inspection of the epoxy primer. Your coating company will have recoat windows based on temperature and humidity, if you go beyond will need to sand it. If you have any doubts about the primer, now is the time to address it. Adhesion test, solvent resistance test, coating thickness meter would all be reasonable, given how much work you have done so far. The outer corners of metal is place where coating thickness is thinnest. We brush "stripe coat" those first then spray everything.

 

I have never seen this much pinholes in paints before either, and btw I only brush and roll the paint, so no contaminations possible from a paint gun.

The paint was a small pack of 750ml total with a mixing ratio of 2:1, but as it came in a pack I just dumped the catalyst into the base and mixed it up.
At first I tried it without any reducer and that went on a little better but a too heavy coat, so I added some reducer and that made to problem worse.

I have put down 3 coats of rolled Hempel Hempadur 15570 epoxy primer with a coarse roller and without thinning the primer, so the paint thickness should be fine.

I only want to cover the floor with polyurethane for a nicer finish and to make is easier to spot dirt and corrosion in the future.

Now that I think about it, the reducer I used was for an earlier version of the paint where the mixing ratio was 4:1, where’s as it is now 2:1, so that could contribute to the problem as well if they changed the reducer too.

Anyway all the poly paint is washed off again, and I’ll let the primer cure some more and try again in a bit warmer weather.

The steel was 3-4 degrees and air was 8-1o with a humidity around 60-65%, so too close to the dew point I think.
I could also dissolve the primer a little bit with the solvent used for washing off the paint, so it was clearly not 100% cured either.

I’ll put it down to being too impatient

 

Quite often the solvents have high VOC's. Make sure that you read the health warning on it. A make up air, ie fan, explosion proof will help.
And if you decide to say use say a 3m cartridge mask, ensure that you look at the specifications of the cartridges as there are different cartridges for different chemicals. A particulate style mask, ie covid mask will not remove the VOC's

 

I actually have 2.5 liters of white epoxy primer (although a different brand) that I could use instead of the polyurethane.
That would give me a nice white dull finish under the floor for easier inspection and might be a better choice as it should have no problems with adhesion between the two epoxy primers.

Do you think this is a better solution ?

I use a cartridge mask with filters for solvents.

 

LP has some great qualities and downsides. If you ever expect water to pool up under the flooring then LP would not be a good choice, it will blister. Epoxy primers will not. LP is much better restisting stains. Epoxy is easier to recoat. My number one choice for bilge areas is old school- coal tar epoxy. Great chemical resistance and durability.

Switching brands always increases risk of coating failures, but I think you are correct in assuming epoxy primers are likely to adhere well to each other. Never had an issue with that.

Successful coating systems are about the preparation, then following recoat windows and film thickness instructions exactly. Hempel is well documented, so you will do well with them.