Eliminating Print Through

Wow, I can't stop with the questions. Sorry folks.

I'm looking at putting the outside fairing/sealing bog layer on an infused hull that's sitting in the shop right now. I'm skipping around because I am waiting on:

*Hull foam (Jan 16)
*Beam foam (this week, I hope)
*Figuring out how to do the dagger board trunks correctly, moving on while thinking

So, that leaves me with a final sealing and fairing above some infused fiberglass with a rough surface created by Econoply E.

How does one properly do this next layer of goo so that print through doesn't happen?

It's epoxy and what... really runny microballoons or really stiff microballoons?

Any other tips for eliminating print through?

 

Eliminating print thru is a science. For small craft, post curing at low temp..say 110 f is effective.

 

That's all you do? No need to worry about the weave of the fabric being telegraphed through the epoxy and making it to the paint?

 

Dont know how you postcure a hull like yours. Ive posted cured small craft in the bright sun covered in a black plastic tent. Perhaps electric blankets would work. The kind that you use to heat concrete work in freezing climates. http://powerblanket.com/

Post curing the epoxy filler is important if you want the wet look.

You can always simply fair and paint...then two years down range...re fair and paint after the filler has naturally post cured

 

I can post cure, no problem, but that's it?

Don't I have to do anything before that?

I have an infused surface with peel ply roughness to it and weave showing through.

What comes next? I'm thinking post curing isn't it. How will that eliminate the weave?

 

Fairing compound fills the weave. Postcure the fairing compound.

 

That is not so bad at all. Thanks, Michael. I can't believe it is so easy. I thought it was very difficult to get rid of the weave. No idea where I picked that up, but it would seem to work.

 

Indeed, apply fairing compound now. Use a thickness which you can work with. Not too runny, not too stiff/dry. Experiment with that. You will develop an eye for it.

Fair whatever you can fair, then apply a coat of epoxy (or more) to protect the fairing compound, and finish with a sanding primer. Depending on your possibilities, I would wait for the complete boat to be assembled before postcuring, but I understand the size could be a challenge (in your area, summer and black tarp are your friend).

After postcure, do a final sanding to remove the inevitable print which was caused by postcuring, then you are ready for painting.

The nice thing about applying a sanding primer is that all surfaces are the same colour again, not patchy and blotchy. This is a big boost for the mind.

 

Concerning fairing compound. Im not a boatbuilder, I fix broken toys..

I never have much luck by mixing my own bog. Get the ratio of filler slightly different between batchs and you will be working with bog of different density and different cure behavior. This means Print thru and fairing issues.
I prefer to purchase the pre mix, one to one, bog. Proset and Awlgrip make tasty easy to work with bog. Ask your local epoxy provider for bog recomendations.

Even with pre mix bog ,mix ratio is important. Develope an accurate mixing routine in your shop.

 

On my own projects I just used the epoxy I was building with mixed with Qcells
then highbuild primer (I use amercoat cc24)
and then topcoat (I use amercoat 450)



Print through has never been an issue
Never done any post curing either

 

Ah ha!

Thanks, Sabahcat!

My designer talks about using the high build primer, but starts there in his discussion. I wasn't sure how to get from infused hull surface up to the primer.

Are microballoons and Q-cells interchangeable as a fairing compound before the high build? Will both keep the print through at bay?

PS: That's a gorgeous paint job.

 

It is not the fairing compound that prevents print though, it is the post cure cycle.

Epoxy has a certain temperature at which the behaviour goes from glass-like to rubber-like. Basicly you do not want to reach the rubber like state. This transition temperature is called "Glass Transition Temperature (Tg)". Many (measurable) things happen at this Tg, which make it relatively easy to measure. (in a lab). The rubber like state is one, but also the expansion rate changes at that temperature, and the amount of energy needed to raise the temperature of a sample.

When RT cured, epoxy usually has a Tg of 45 degrees C or so (sorry, I am metric). The fun thing is that when you cure at say 40 degrees C, the Tg will raise to about 55 degrees. When curing at 50 degrees, the Tg will raise to 65 degrees. With a curing cycle like that, you will never "run over" the Tg, so you will never see the rubbery state. (rubbery is an excaggeration, it will not be really rubbery). However, it is rubbery enough to develop print.

Imagine a non-post cured epoxy, painted black, nice and shiny, and put in the Florida sun. The temperature will raise rapidly, runs over the Tg, and print through develops. Also, the epoxy post cures, and reaches a higher Tg. Next time it sees the sun, it will not develop print anymore. (although it already has print from the first exposure).

Now what you want is apply all epoxy, (laminate and fairing compound) then postcure. This way you know that you have a decent Tg, and sun exposure will not cause print. During the post cure process your boat might develop print at places. That is why you do not want to post-cure after painting. You can post-cure as long as you have not done your final finish sanding, to remove eventual print.

One more thing: Every epoxy type has an "ultimate Tg" This temperature is the maximum Tg you will reach, no matter how high your cure temperature is. Check the datasheet of the epoxy you are using (also of pre-made fairing compound) to find the ultimate Tg.

On cure schedules:
We usually recommend a temperature raise of max 10C per hour, to give the epoxy the opportunity to cure properly. This way you will not overrun your Tg. This is, if you can control your temperature. Witht he black tarp procedure, I suggest placing the black tarp in the evening, so curing starts early morning, when it is still cold.

The higher your reached temperature, the faster you reach ultimate Tg. Many epoxy samples are cured 2 hours at 120C, to develop a Tg of some 80 degrees or so. Of course the ultimate Tg is overrun dramatically, but when cooled, properties are not affected. This way the lab operator knows for sure the sample is cured 100%, and can do accurate measurements. He (or she) does not care about print-through.

many epoxy suppliers have suggested cure cycles. If not, this usually works:
12H at temp X gives a Tg of X+15 degreesC
18H at temp X gives a Tg of X+20 degreesC
24H at temp X gives a Tg of X+21 degreesC

So at least try and keep the highest possible temperature for at least 18 hours (in one go, or divided over more days, so order a couple of sunny days!

 

How would you post cure a big square catamaran with verticle and horizontal surfaces in a home workshop ? Plastic tent , propane heaters and circulation fans ? How would you insure, prove, even heating of the hull.

Do you post cure with the primer, high build already applied ?

 

I would not cure with the high build already on, although I would consult with the supplier of the high build as well.

How to do it? Completely up to your imagination and possibilities. Hire some diesel-powered heaters, indirect heaters please, so the thing just exhausts clean, hot air. Use fans and ducting to spread the heat.

Being in a warm environment helps a lot. It can even be possible to reach a decent temperature using black tarp and the sun.

If possible, insulate as much as you can, the floor pulls a lot of heat away, so insulate that if you can as well.

 

I'm in a location that reaches 41.1C 106F on a regular basis in the summer. The epoxy manufacturers said I didn't even need to post cure.

However, if I do (to deal with print through possibilities) I'll just rent a heater for a day and fire it off on one of the hottest days in my building.

Despite the hardship of the heat, I am convinced it's easier to build a boat in a tropical or semi-tropical climate than up in a temperate climate. Cheaper too. A little air conditioning is a lot cheaper than heat.