Clear coating prepreg

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#1

Hi guys.

This is more brain picking than anything else.
I use VTF261 as a surface (out of autoclave). For clear coating, I normally hit it with one coat, sand, then repeat as many times as necessary until the part is flat. This is getting tedious and so time consuming every time I make a part it makes less and less sense to clear it. However, it has to be cleared for UV, gloss, and most of all, depth.

Normally, parts come out quite well out of the mould. Very smooth and to the naked eye, pinhole free:

However, when clear coated, it’s like a rash:

Tried cleaning with both mould cleaner, acetone and thinners in case it’s a release agent issue. Nothing.
Tried sanding it quite a bit, no difference.
I believe it’s the carbon that’s drinking the lacquer through the pinholes.

Basically, does anyone have a quick and easy way of clearing OOA prepreg that doesn’t involve 5 coats of clear and days of sanding?

#2

Try zpreg VTF 261 :wink:
Are you sure you cant see any pinholes before painting? they seem to be there. What cureschedule are you using?

To check on surface defects, get some light colored dust from sanding, and rub it over the part. But you’ll probably know that, spending so much time sanding…

#3

That is VTF 261…

Those 2 dots by the light on the first photo are part of the reflection.

Cure schedule is 3 hours at 65 followed by 10 hours at 75. Ramping at 0.5 degrees / min with a PID.
That’s after a 12 hour debulk.

#4

It is nearly impossible to get a pinhole free part with a OOA prepreg. So there will be very small pinholes.

#5

Thats weird, the painter I worked with never had issues with pinholes. Sometimes some air between the glass and carbon, because air evacuation wasn’t good enough, but never pinholes on the surface.

#6

Thanks DDCompound, thats encouraging, lol.

Susho, what are your moulds made of?

#7

They where polyester, epoxy/carbon wet layup, and some prepreg.
Never had pinholes in the glass layer.
Did some parts with ooa prepreg from delta too, after some fiddling around with the cureschedule there where pinhole free, even without the glass layer like zpreg has.

After I left the factory they changed suppliers too, and I haven’t seen clear coated parts after that, only some small parts. I wonder why…

What are your moulds? looks like a polyester/vinylester rapid tooling system? You probably have post cured them already, and they have been on elevated temperature too… One thing I could think of is outgassing of the resin, which I know screws up paint systems causing blisters on poly parts who get (too) hot.

#8

They are Nord GC206 VE gel with Optimold.

What was the cure schedule you used for Deltapreg? They are sending me a low temp sample soon, so hopefully I can imitate your results.

I have post cured them, but Ive just thrown them back the oven , in case they werent there for long enough.

#9

Make sure your first layer of prepreg is not too wet. It provides an airpath. Extend the prepreg out of the stack a bit. Also for air transport.

Debulk the prepreg very thoroughly before tossing it into the oven. At least 30 minutes. Get a vacuum as low as you can. The complete stack should lose its air before heating things.

#10

VTF261 is dry fibre with a resin film. The glass tows in the pics are there for air evac. Vac at least 96% for VTF 261, but thats all in the docs. With this cureschedule the air should have plenty of time while the resin impregnates the fibre.
I dont remember the exact cure schedule, but I remember a slow ramp with dwells at 65 and 85 degrees for half an hour on small moulds. and a higher cure temp then the VTF261 series. Those I normally cured at 85. I can’t remember which delta series I used… But slow ramp till the resin starts flowing and some dwells to even out all the temperature differences is key for every ooa prepreg, together with air evacuation.

#11

I debulk for far longer than that. Overnight sometimes.
I never cook until I have at least -0.95bar and a 30 min drop test with less than 500mbar drop.

Have been advised to insulate the exposed side of the part, so that the mould heats up first, getting the surface warm before the backing plies. This made little difference. Pinholes are not visible to the naked eye. Only lacquer exposes them.

#12

I dont have any experience with prepreg so i cant offer a solution to the source of your problem. But i do unfortunately have experience with pinholes :o i can suggest a band aid solution till you sort your issue at the source.

Instead of spraying 5 coats of clear and sanding between, you could use a product like Car System Carbo Spray, basically its like a transparent spray putty. If your pinholes are nearly invisible like you suggest, this stuff should fill them up being quite viscous. Then after rub down and prep for final clear coat.

Its worked ok for me on some overlay jobs that had terrible pin holes. Larger holes needed an extra thick coat i noticed. They also sell putties and stuff too, which im yet to use.

#13

If pinholes are obvious thing , then sometimes help shooting few light mist of clear, within few minutes each other, and then shoot fully layer

#14

I’d shoot the mold with clear before layup. Deltron 2K’s can handle those temps.

#15

Thanks, I have contacted the UK distributor of Car System products, they said they don’t stock it, unfortunately. It does sound like the product to use though.

#16

I thought about this. I thought pinholes would still form under the clear?

#17

I used this technique but if you get pinholes in the part they will be very noticable under the clear coat as white spots. and you have to take all the clear out to solve this problem!

#18

Often pinholes are opened up when the part is sanded. Clear-coating the mold eliminates any need for sanding. If the pinholes are due to layup errors/issues then the repair process is miserable regardless of the finishing process.

#19

but this thread is about issues/errors in the layup so clear coating the mould won´t solve the problem in this case, it will make it worse, in my noob opinion of course :slight_smile:

#20

There isn’t any layup issues, as DDCompound said, pinholes in OOA prepreg is a given.