Base coat ruined from?

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PostPosted: Sun Dec 15, 2019 1:46 pm
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Hello people,

I sprayed a PPG acrylic sealer on fenders and bumpers, and everything went perfect! I let parts cured and after a week I scuffed them with a scuff pad. I cleaned everything with the Rust Oleoum wax and tar removal.
Immediately after I sprayed the PPG acrylic base coat these cracks (see pictures) showed up.
It happened before with other paints and other products. There was no way to sand and redo it still came up again.
I had a temperature of 70 in the garage.
Anybody knows what am I doing wrong and how to fix it? It's very frustrating.
Thank you all for an eventual help.
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PostPosted: Sun Dec 15, 2019 2:46 pm
This is a reaction between the thinners in your applied coat and something underneath that is sensitive, usually a two pack product, often a primer. It is exacerbated usually by the thinners in basecoat which tend to be a bit more aggressive, which is why it didn't happen with your sealer coat, and heavy applications of each coat which allow the wet paint to soak on the surface and react, instead of flashing off quickly

Solutions are:
1. Sand back all the substrate to bare metal and start again, using known, compatible products.
Or:
2. Sand back until it feels smooth then apply multiple, extremely light coats, allowing each coat to flash fully, even playing air from your gun to help the thinners evaporate before they can react. You may need like 10 of these really light, almost not there, coats to build up a layer. Allow to fully dry, sand and apply your basecoat, again in light coats, but not as light as you used for the primer. At about 20C allow 15 minutes between each coat and then 30 minutes if you have to go beyond 3 coats (which you shouldn't). Use a faster thinner in both the primer/sealer and the basecoat to encourage it to evaporate quickly.

Tip: Tint your sealer to the appropriate shade for your basecoat. This will help you get coverage without too many coats.

Don't apply heavy, overwet coats.
Chris



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PostPosted: Sun Dec 15, 2019 4:50 pm
Thank you very much NFT5!

I will follow your directions! Clear and perfect!
I have a question more. How long the paint can sit in the gun? I am a beginner about painting. I thought, if I have to wait too long between coats the gun can get gummed?
Thanks again!

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PostPosted: Sun Dec 15, 2019 7:16 pm
Robbyvenetian wrote:if I have to wait too long between coats the gun can get gummed?


Depends on the gun and the paint.

A suction pot type gun can have acrylic primer in it almost constantly - lots of shops used to and some still do, this. Just give it a swirl around and paint. Not really recommended, though.

In a gravity feed gun the heavier parts of the paint will fall to the bottom of the cup and build up there and in the passages of the gun. Best practice is to clean the gun after applying your 2 or 3 coats or within, let's say, an hour. For two pack products this is absolutely essential - pot life may only be an hour, so must be cleaned immediately.

There are some new speed clears that build up between the needle and fluid tip within 5 minutes. For these the gun must be cleaned, or at least flushed out, immediately after each coat. How do I know this? Want to come and clean the 2 guns I have sitting on my bench, in which this speed clear went off in only 10 minutes. :(

Clean your gun(s) thoroughly each time. Dispose of excess paint then add about 10% of the cup capacity with thinners or gun wash. Swish around well and spray some of this through the gun. Dispose of the rest turning the gun upside down and pulling the trigger to release what's in the gun. Repeat with fresh thinners/gun wash, this time flushing through the gun into your slops tin. Remove aircap and clean thoroughly, checking all holes and flushing with clean thinners. Remove fluid tip (remembering to pull trigger to hold back needle) and flush with clean thinners. You'll be surprised how much colour remains in here, even after flushing. Flush and use a soft brush to remove any paint on cup and lid (including anti-drip reservoir/cap) and needle that is visible when trigger pulled. On a Devilbiss gun, remove pot and clean in the neck of the gun. Removing the needle is optional - I do mine about once a month for the guns that I use pretty much daily and I lubricate at the same time.

:happy: A lot of that didn't have much to do with what you asked, but hopefully should be useful for you, or someone else, at some point. Sometimes I just ramble.... :knockout:

Ok, time's up waiting for that last coat of base to flash, so have to get back to work.
Chris



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PostPosted: Sun Dec 15, 2019 11:41 pm
Thank you very much! I could not have better answer than yours, and fast! I
ll try everything you suggested and I will post here the results!



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PostPosted: Mon Dec 16, 2019 2:16 pm
excilent info above^^, but if you could post the products you used we could maybe pin it down to that specific one. then you could use something more compatible with your other products
Jay D.
they say my name is Jay



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PostPosted: Mon Dec 16, 2019 7:33 pm
badsix wrote:excilent info above^^, but if you could post the products you used we could maybe pin it down to that specific one. then you could use something more compatible with your other products
Jay D.



Hello badsix. Thank you for your support.

I used PPG products. From their page everything is compatible. This is the list

JP315 1K Acrylic Sealer with JR506 Medium

JBP SHOP-LINE® Plus Acrylic Urethane Basecoat with JR506 Medium

I wonder if it's better to avoid the Rust Oleum wax and tar and clean with soap and water.

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PostPosted: Mon Dec 16, 2019 8:45 pm
Although it's 1K, that sealer is designed for wet-on-wet and after 72 hours needs to be sanded and re-applied before basecoat.

So, it seems your application method was incorrect. It really is important to follow the TDS to the letter with these new products. They're awesome, but not so forgiving.

Sand it all back until smooth. Re-apply a single wet coat of the sealer, wait 15 minutes and apply your basecoat.

Keep your coats light. The sealer should be only just wet on application. Basecoats should be light and even. I note there's quite a bit of texture in that picture, which would indicate overwet application.
Chris



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PostPosted: Tue Dec 17, 2019 4:43 am
could that wax and tar remover be the problem? or the time between using it and spraying?

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PostPosted: Tue Dec 17, 2019 8:54 am
^^^^^^^^^^^That was the first thing I was thinking....Wouldn't be surprised if that stuff had some recycled thinners in it which could be very "hot." Your working in PPG so why not pick up some Acryli-Clean?
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