Primer Sprayed at too low pressure

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PostPosted: Wed Oct 13, 2021 11:01 am
I'm working through my first paint project. Everything appeared to be going well until I discovered that I sprayed the primer using too low of a pressure. I think I may have misread the doc and sprayed at about 10psi at the gun. I'm thinking that it should have been more like 20psi.

Did I misread the document?
(https://www.axalta.com/content/dam/NA/H ... 10-Eng.pdf).

The primer looks fine and I'll be sanding that down this afternoon in preparation for base/clear. What problems, other than having a bit more sanding to smooth things out, could result from this?

Thanks so much

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PostPosted: Wed Oct 13, 2021 11:47 am
So, yeah, I think some technical writer dropped the ball on that one. That 7-9 psi for HVLP is probably referring to "cap pressure" which is a figure almost no one can use or measure in practical spraying. What does your gun tech. sheet say about pressures for spraying primer?
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PostPosted: Wed Oct 13, 2021 12:34 pm
Yeah, I should have known better but since it's my first try, I didn't question it.

As far as I can see, there's nothing specific in the manual. The closest mention of pressure is "30 PSI inlet pressure provides 10 PSI at the air cap."

https://www.autorefinishdevilbiss.com/D ... 0&TabId=86

The overspray is definitely going to need to be sanded smooth but the main spray areas feel alright (to an untrained hand). The main areas aren't unreasonably rough. It _seems_ like I should be able to sand it smooth....fingers crossed.

Is there a chance of something worse like failure to adhere?
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PostPosted: Wed Oct 13, 2021 1:52 pm
That means you should set the air pressure at the gun (with the trigger pulled full open) to 30 psi in order to get 10 psi at the cap.

Also remember that every gun has a CFM (SCFM) requirement as well. It will generally say something like 12 CFM @ 30 PSI for a particular tip set.

Just read your tech sheet and it states: "FULL SIZE GUN: 30 PSI inlet pressure provides 10 PSI at the air cap. Consumes 13 CFM"

Your compressor will need to produce at least that much CFM in order for the gun to spray correctly. It is usually best to get a compressor that exceeds the CFM requirements of your spray guns and air tools by 10% or more.
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PostPosted: Wed Oct 13, 2021 2:29 pm
Thanks for the explanation. I, sheepishly, get it now. :knockout:
The question remains, though, have I completely goofed this job or am I just in for a tonne of sanding?
Will the primer fail to adhere to the surface over time or any other such thing?
Thanks

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PostPosted: Wed Oct 13, 2021 3:15 pm
gprs wrote:Thanks for the explanation. I, sheepishly, get it now. :knockout:
The question remains, though, have I completely goofed this job or am I just in for a tonne of sanding?
Will the primer fail to adhere to the surface over time or any other such thing?
Thanks


As long as you mixed the primer correctly, you should be fine. --- Though DEFINATELY get more opinions from this site's experts as I am far from one, kind of the opposite, but I have done the same thing (more than once...)

Depending on how bad it is, block it out and reshoot some more primer. I am still hoping for the day that I put on more than I sand off.

Use guide coat.

Are you planning on doing more painting as a hobby? If so, sell that Starting Line gun and get something better.
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PostPosted: Wed Oct 13, 2021 10:01 pm
you should be good other than it went on rough and needs a lot of sanding. cut it down with some 180 . you probably learned something here. when you could see it was not going on smooth the first thing you should have looked at is my air pressure. if your not sure what it should be turn it up or down and try it. it will get better or worse you need to experiment some.
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PostPosted: Thu Oct 14, 2021 6:35 am
Thanks, everyone. I have definitely learned a tonne during this process.
I didn't notice anything wrong while I was spraying but, since this is my absolute first attempt, I really had nothing to compare it with.

I only realized there was a problem when I was re-re-reading the sheets for the base and seeing a reference to air pressure at the gun _and_ at the cap and the "at the cap" pressure matched the single pressure provided on the primer sheet which, even at the time I suspected was low. Who was I to argue with the product sheet though?

A very large lesson indeed. Paid in full, now, with a few hours of sanding.
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PostPosted: Thu Oct 14, 2021 10:57 am
What's nice about the primer stage is you will get plenty of practice at block sanding and spraying more primer.

To get your panels nice and straight, you want to use coarse grit on the initial block sanding.
Do not press real hard on the sanding block, let the paper do the cutting and when it quits, change it.
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PostPosted: Thu Oct 14, 2021 4:13 pm
Thanks. I'll do that next time. I was concerned that I might create scratches that I'd have to fill so I blocked it all out with 400. It took time, effort, coffee, and paper but the result is pretty smooth.
Thanks again, everyone!

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