I do carbon bonnet with infusion process without gelcoat on tool surface
and this is the first time for painting process.
I use 2k polyurethane clear coat and I notice some spot on the surface as
picture.
I think this happen because I didn’t apply gelcoat on tool surface before infusion.
Please advice 
A bigger pictures whould help to determine what is wrong.
Looks like the paint is fisheyeing. How did you prep the surface?
http://pc.dupont.com/dpc/en/US/html/visitor/s/trouble/PDSG_Fisheyes.html
Origin and Potential Causes:
Improper or insufficient surface cleaning or preparation. Many waxes and polishes contain silicone, the most common cause of fish eyes. Silicones adhere firmly to the paint film and require extra effort for their removal. Even small quantities in sanding dust, rags can cause this type of failure.
Effects of the old finish or previous repair. The old finish or previous repair may contain excessive amounts of silicone from additives used during their application. Usually solvent wiping will not remove embedded silicone.
Contamination of air supply, by water or oil.
Oil, wax, grease or silicone contamination.
Use of silicone-containing polishes or aerosol sprays (e.g.: interior cleaners or dressings) in proximity to the spray area.
Here are the steps I prep surface after demolding
-sand #240
-sand #400
-clean surface with acetone using foggy
Please also advice how to repair this. Should I sand it off and re-paint ?
Sanding with 1000 or more is better for a clear coat. Don’t clear surface with acetone but with some degreaser that is made for it, acetone is to aggressive.
Sand te clear down untill you don’t have the holes anymore. So that probably completely.
Michiel, thanks so much
what degreaser do you use ?
Use wax and grease remover, NOT a degreaser. Personally I have used acetone before and it is fine, but wax and grease remover is the intended product.
Mmunited, in future I recommend you wash the part with soap and water when you remove it from the mould, and give it a quick wipe with acetone or MEK or wax and grease remover. THEN do your sanding. Issue is, if you start sanding it straight away, you can actually rub any transferred release agent deeper into the surface, causing these problems. I also agree with Michiel regarding the grit level.
This could also be pin holes in the surface. Infusion may not always be perfect and since the part has no mould coating you could have porosity which typically does not fill when sprayed over.
How different between pin hole and fish eye ?
I need to know what is exactly cause pin hole or fish eye.
No no, it’s fish-eyeing, but pin holes can cause it.
I guess I always consider a fisheye something caused from surface contamination where the paint just won’t flow over that spot.
A pin hole creates the same sort of look as the paint just won’t flow into the hole. If it is porosity then you may still be able to see the pin hole.
I think if you really want to ace your paint job then it is best/safest to use an in mould coating. Maybe a clear two pack paint or clear gelcoat like duratec? You can then safely clear coat over this again if needed.
Thanks much :))
If you are getting pinholes or small air bubbles trapped in your composite , an in-mold coating or clear will only trap them visibly below your surface . If you were only getting fisheyes from a post finishing process then I agree the in-mold is a great time-saver .
When ever I do an infusion I either coat the mold with Duratech top coat, or I Duratech top coat the part after demold and trim. I always use Duratech top coat because it eliminates any tiny pin holes, (if any) and makes clear caoting a breeze.
From my experience never clear coat anything over 800 grit. Anything higher and the sand scratches aren’t coarse enough to grab the clear coat well. You can get easier delamination. Thats just my experience and what I’ve been told / discussing with colleagues over the past 7 years.
I personally clear coat over 600 or 400 sometimes. I also use a high build high solid’s clear coat, (2:1) so I don’t have to worry about seeing the sand scratches through the clear coat.
I also use either PPG DC3000 or the top of the line PPG, (I forgot the name) so I get very little shrinkage.
However, if I use the DC3000, (speed clear) I make sure I sand to 600 because the ultra fast clear will have more shrink than the 18 hour cure clear. Same applies for force curing.
To clean the parts I usually power wash first or wash with Acetone. After power washing and drying, or acetone wash, I wipe the parts down with Naptha, prep-all, or denatured alcohol. Urethane reducer works also if you have it and you are in a pinch.
Acetone and Lacquer thinner leaves a very thin film on the parts I hear.
John
Those imperfections are caused by pin-holes in the laminate. Are they located right over a weave intersection in the fabric?
Way back in the day, I had the same problem with my first carbon parts that were sprayed. I sanded and cleaned the parts with all kinds of cleaners, cleaned my spray booth, put solvent in my air lines, got new airlines, got a very expensive air filter on my compressor, tried a new gun, changed clear-coats, tried fish-eye inhibitors in the clear-coat, and changed the release agent. None of these things fixed the problem. I spent heaps of money and time trying to eliminate fish-eyes only to discover that they were pinholes.
I would bet my composites career on the fact that those imperfections are caused by pinholes. The more you sand the worse the problem will become.
My process was this…before I used an IMC.
I would typically only have to clean the gun once for the whole process if I force cured the first application. The pot life of many clears is around 4 hours. I would shoot, cure, sand, and shoot within the original pot life.
I moved my entire production to IMC techniques to avoid the whole problem and significantly speed up the finishing process. Perfect molds will make a perfect finish.
So, what should I do if I want to avoid these imperfections.
Sanding #400 is enough ? or #1,000 should be better for prepping surface ?
Please advise.
I have to disagree with this a little bit as I feel that often bubbles appear due to the resin not flowing across the mould surface well due to release agents on it. When you have an in mould coating the resins flow and attach to the coating better that it can on a bare mould so trapped air is reduced.
Fasta, where do you get your IMC from in Perth? Nuplex? Can you only get it in large quantities?
That’s fine , I only share what I have experienced myself . We’re only offering data for him to experiment with as he develops a process that suits him . 