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May 24, 2019, 02:36 PM
Registered User
eitanro's Avatar
Thread OP
Help!

"Fisheyes" when painting molds


Hi all,
I'm trying to paint urethane paint on a mold, but for some reason I have very bad fisheyes (I use a water trap and wax the mold with this release agent).
Any clue/idea how I can prevent this from happening again?
Thanks,
Eitan
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May 24, 2019, 05:57 PM
Slow builder
_AL_'s Avatar
Thin dry coats. Don't try to paint it like a car or the paint will fisheye like it has silicone contamination.
Very light dry coats until you have coverage.

Al
May 24, 2019, 06:21 PM
Entropy is happening!
Jim.Thompson's Avatar
High probability of silicone contamination.
Wash with Prepsol before doing the same again.

Also, follow Al's suggestion above. If fisheyes start, stop painting, clean/strip and start again. The voids will not "fill", not without an lot of excess paint.
May 24, 2019, 07:21 PM
Registered User
Roguedog's Avatar
Agreed. some type of contamination.

Something not mentioned is make sure the wax you are using does not have any detergents in it.

I use a pure carnauba wax. It's hard to find one without detergents in it
.

The detergent is to remove dirt, oxidation etc on painted surfaces. Would act just like silicone contamination if used. Every see the Dawn commercial when the dribble the Dawn into the oil filled water. It disperses the oil.

If the detergent in the wax on left on the surface it will act the same way as silicone and disperse the paint.
May 24, 2019, 10:13 PM
Registered User
I believe that the key to avoid fisheyes when painting in molds is to spay dry and very light and several passes with the pray gun. Even for "professional" use I prefer to use a "touchup" spay gun as it is easier to finely adjust the spray parameters (air pressure, paint flow and spray pattern) as they all play a role. I prefer to use a "fast" hardener and even, some time "fast" thinner. The key is to spray thin and light and cover the surface with several passes over.
May 25, 2019, 06:41 AM
Registered User
eitanro's Avatar
Thread OP
I'n always paint in 3-4 thin layers... I'm not new in this, but I have never seen such a bad result like I see with the urethane paint
May 25, 2019, 06:56 AM
Entropy is happening!
Jim.Thompson's Avatar
Quote:
Originally Posted by fnev
I believe that the key to avoid fisheyes when painting in molds is to spay dry and very light and several passes with the pray gun.....................................
While that is good practice, it will not avoid fisheyes when silicone contamination is present.
If you doubt this, do a test.
Guess how I know?......................
May 25, 2019, 08:26 PM
One Idiot is plenty...
Dbox's Avatar
This wax is NOT compatible with ANY paint.
Yuri.
May 25, 2019, 10:50 PM
Scott
Pylonracr's Avatar
I never paint in the molds, and I never use Semi's. I am a wax and PVA guy. I would strip the mold using wax and grease remover from the paint store. wax with 6 coats of Partall wax, put a coat of PVA on, and paint. Easy, unless you plan on masking and removing paint lines. For a single color, you are golden.

If you want to mask and paint colors in the mold you will need to use a semi, and you are on your own,,,,,,,,,,,,,

Scott
May 26, 2019, 12:02 AM
Entropy is happening!
Jim.Thompson's Avatar
Quote:
Originally Posted by Pylonracr
I never paint in the molds, ..........
Wwwhhat?
Why not Scott?
For heavens sake man, catch up!

Quote:
and I never use Semi's. I am a wax and PVA guy. .........................
Half right track there Scott! Skip the PVA (in seasoned moulds only) and then you are, as you say: "Golden"!.............. PVA is for green moulds and makes for stippled finish, at best.

Quote:
If you want to mask and paint colors in the mold you will need to use a semi, and you are on your own,,,,,,,,,,,,,

Scott
Only too happy to be on my own, as you well know by now Scott. If you don't know by now you have not been attending!

I mask in the mould using wax only. With some difficulty and the odd curse or two!
But hey; the neighbors have not complained yet!
So, I figure: what's the problem?
What, me worry?

Jim.
May 26, 2019, 12:59 AM
Registered User
Roguedog's Avatar
So I'd like to mention that when you hear "PVA is for green molds" Rexco's own tds is referring to polyester molds not epoxy molds.

The issue is striclty a phenomena that happens with polyester molds only when using polyester resin to make a part from the polyester mold. The problem is styrene migration. Epoxy resin does not have this problem.

When using epoxy as the resin for the mold it is completely inert after 24 hours. So you don't have the same crosslinking problem unless of course you added to much epoxy hardener and if that's the case you may or may not end up with a completely cure part or mold. If the epoxy was mixed correctly the resulting epoxy composite should now be a plastic, e.g. inert.

It took me long time to understand this difference when i first learned about PVA. Rexco's FAQ for PartAll clearly says --
If mold is new or reconditioned, then you should definitely use a PVA such as
PARTALL® Film #10 or PARTALL® Coverall Film to prevent styrene migration. If mold is seasoned, then the use of PVA is not necessary except as additional insurance against sticking as long as wax is applied properly and with sufficient frequency.
The preceding statment is refering to molds and parts made from polyester resin not epoxy resin.

It also clearly says before that -
if you do not want the part to stick to and possibly ruin your mold, then use PVA.
Read Rexco's FAQ which is attached.

Of course if you're looking for short cuts to have a finished paint job in the mold by all means just wax your mold, spray the paint, then do the layup. If you're worried about your part sticking use PVA and paint it after the part is made.
Last edited by Roguedog; May 26, 2019 at 01:11 AM.
May 26, 2019, 04:13 AM
plane destroyer and builder
skybattle's Avatar
You need to have a lot of patience... waiting some minutes between coat and coat as other people have told you. Apart from other problems, this is the only secret.
May 26, 2019, 11:11 PM
Scott
Pylonracr's Avatar
Well Jim, I guess I will answer your questions since you asked so nicely..
1) Paint in the mold. I don't paint in the mold because, well, UUUMMM, EERR I don't paint in the mold. No real reason. I just don't. Most of what I do is custom 1 off components. The one thing that would be really nice to paint in the mold is EF-1 cowls. The problem is that I make them on a male mold, so if I painted the mold the paint would end up on the inside and not do me a lick of good. One day I will paint in the mold just to tell you that I did it.....

2) Wax, PVA, Semi's. Easy. Wax and PVA works for me. If I did production race planes with cool guy graphics painted in the mold, Semi permanent mold release would be the way to go. I don't. Most of what I do is - Make plug, make mold off of plug, make component. This means that I work almost exclusively with green molds. I always make my mold with the same resin I will use on the final component. We all know that wax alone will not provide sufficient release on a green mold, so I use PVA. Shawn taught me a trick to apply PVA with a foam craft brush. With very little practice I can now put down a perfect glass smooth layer of PVA on a mold. The cost is negligible, and the time is not a factor. It works for me and so even when I use a seasoned mold I use PVA. I have a system that works for me and I do it the same way. I am sure that there are times that I don't need the PVA, but to me it is not worth finding out that I am wrong......
Your mileage may vary.....

Scott
May 26, 2019, 11:25 PM
Entropy is happening!
Jim.Thompson's Avatar
Quote:
Originally Posted by Pylonracr
..................... With very little practice I can now put down a perfect glass smooth layer of PVA on a mold. .........................

Scott
Scott,

This is where our respective skill sets differ radically.
I have never been able to lay down a film of PVA that was "flat" in the sense that spray painters use that term. It has always been stippled, or otherwise not a spray paint quality finish.
If you can, as you claim, (and I have not reason to doubt you), then I have the utmost admiration for your skill.
I should also add; others have posted on the composite forums that they achieve a "paint in the mould" quality finish using PVA.
For me, where I am located in relative isolation, (except for these forums), this standard of achievement is illusory.
And finally, I should add: I am far from being the only composite construction modeller that has formed this conclusion.

Until I (ever) achieve the same level of skill in the application of PVA that you report, I will stick too (no pun intended! ) wax only.

Jim.
May 27, 2019, 12:14 AM
Registered User
Roguedog's Avatar
It took me about a 1/3 of a gallon or PVA to learn on to spray it so it is wet look mirror finish when dry .

Where I'm located is very arid during the summer and I sometimes have to add 50 % water to the PVA to get it too spray correctly. It's not possible to spray it straight out of the gallon container at any time of the year where I live.

So that requires different ratios of water to PVA depending on the weather.


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