Print-Through

[pauljohn]

I am having a print through problem on a laminate that only shows itself when it is warmed by the sun. The laminate was post-cured to about 170* and has been taken up to that same temp on a regular basis by the sun but still shows print-through above about 120*.

The epoxy laminate is hand-laid and made up of cloth with a layer of surfacing veil and finished with several coats of a polyester primer then two-part poly paint. It is a high quality mirror finish that is flawless at room temp.

I have tried sanding off top-coat, then adding more primer without any success. Does anyone have any ideas what I can do about this?

Thanks for any help.

 

[berkshire]

pauljohn
Have you tried post curing at a higher temperature?
It may be, that the resin you used does not, have a high enough heat deflection curve.
B.E.

 

[Compositepro]

Print-through is caused by the much higher coefficient of thermal expansion of resin compared to glass or carbon fiber. For heat-cured composites print-through usually reduces as temperature is raised.

In your case the resin was probably room temperature cured and the surface was sanded smooth and polished at room temp.
Therefore the surface is mirror smooth at room temp. As temperature increases, the resin in the weave openings expands more than the fiber and the pattern becomes visible. There is not a lot that can be done. The mirror surface shows the most microscopic deviation from flatness. The solution is to cover the laminate with a thick gel coat which makes the print-through less visible but the thicker the gel coat, the less durable it is. It will be prone to cracking. Make the surface less glossy to reduce the visibility of print-through.

 

[pauljohn]

Thanks for input. I will try and raise the temp to see if that does anything, is it possible that laminate a year old could still benfit from this or has the curing process done at this time?

What would you use as a 'gel-coat' on epoxy? Do you think building up the polyester primer (duratec) would help at all? It is not possible to reduce the gloss and I would like to fix this on the existing part and not remake another, I have about 600 hours in it.

 

[berkshire]

Pauljohn,
The polyester primer Duratec is actually a polyester enamel
It is a catalyzed polyester in a solvent vehicle. It cures by evaporation and cross linking, it goes on better than straight gel coat, has better adhesion on epoxy, and provides a halfway decent finish right out of the gun, it only requires a fine finish sanding and polishing. If this is done too soon and solvents continue to evaporate you may get print through later, in which case block sanding what you have, may get rid of the print through.
If Composite pro is right, and he usually is, then block sanding the finish at a higher temperature may get rid of the print through, until it cools down again, then it will re appear.
B.E.

 

[pauljohn]

Thanks again Berkshire for the reply.

OK in laymans terms here is what I understand is going on. Print-through caused by the curing process is done, it is unlikely that heating the laminate up now past 170* will do anything. What is causing the print through now is simply the different rates of expansion that happen when the laminate is heated by the sun and nothing is going to stop it happening.

So my options are finish it at room temp. and accept that when it is outside the sun is going to bring the pattern out, or, finish it when the sun is on it then the print through will come out when it cools down (sort of a reverse pattern). Or I could finish it at a mid-range temp and hope it mitigates the print through at both high and low end temps.

My only other option is to keep covering it with primer and hope that it masks the pattern from coming out in the finish but this would depend on the flexibility and thickness of the primer.

Does that about sum it up?

 

[berkshire]

Pauljohn,
That about sums it up.
If you increase the primer thickness beyond .032" expect to see cracking later.
B.E.

 

[Compositepro]

The way to avoid the problem in the future is to start with a fine weave pattern like 7781(8-harness satin). Low CTE mineral fillers added to the resin will also reduce the CTE of the resin in the weave openings. But fillers will make the resin more brittle.

You might also get some benefit by using a paint designed to stay cooler in sunlight. These are reflective at higher light frequencies and emissive at low frequency so it helps reject heat.

 

[Corrosiondoc]

I used to mfg. custom composite furniture and had the same print thru problem. You are correct about the heat and sun causing the problem but once print thru has occured the gel coat can be surfaced and print thru will NOT reappear.