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Prevent dry over spray on everything?

PostPosted: Sun Sep 08, 2019 10:59 am
by texastravis
I put in my epoxy sealer over my 2k primer Las night. This morning, I found dry over spray all over flat parts of the car. I'm guessing this is from shot over spray as well as settled over spray throughour the night. Is this normal to be covered on dust like this? I was hoping to spray color today but looks like I got some cleaning to do first.

I built a diy quick paint booth with box fans. not surprised they don't move enough air. Is thus going to be a problem when I put on my single stage?

When painting the lip of a jeep like this, how do you prevent over spray in the tub? Should I paint the tub first, let it cure, then shoot the sides?

Re: Prevent dry over spray on everything?

PostPosted: Sun Sep 08, 2019 1:06 pm
by texastravis
I noticed while cleaning that the flat surfaces have a textured rough feel while the vertical panels feel smooth. I will probably scotchbrite the flat panels a bit smoother and just hit them with a few coats of my single stage. Any roughness after painting I will just color sand off. I did add another fan so maybe it wont be as bad.

Are you "at home" painters with cheap booths/garage having the same issue with dry overspray after coating?

Re: Prevent dry over spray on everything?

PostPosted: Sun Sep 08, 2019 4:26 pm
by '68 Coronet R/T
I think we need to go back to some basic questions:

Size, hp and CFM of your compressor?
Brand and tip size of your spray gun?
Settings: Fluid knob turns out from bottom? PSI at the gun with trigger wide open?

Distance from panel when spraying?
How are you spraying the car? (i.e. top to bottom, front to back, into the fans, away from the fans)
Brand of epoxy?
Brand and speed of reducer?
Temperature in booth when spraying?

Re: Prevent dry over spray on everything?

PostPosted: Sun Sep 08, 2019 8:36 pm
by NFT5
Before getting into fine details like gun. pressures, etc., overspray only gets on to panels that aren't masked properly.

If you don't want overspray in a tub:
2018-06-14 20.25.57.jpg

If you don't want overspray on other panels:
2017-10-04 16.22.29.jpg

This is probably a really good example picture. The grilles you see beyond the car are the inlets, so air flows from those to the other end of the booth. My booth moves air at in excess of the required 0.5m/s yet I still masked the whole car, even the parts that are upwind.

Plastic masking rolls are cheap at around $30 for a 5m x 120m roll and masking this way is fairly quick, once you get the hang of it. Zero overspray, or even "booth dust", means that, most of the time, I can simply strip and the car's ready for delivery.

If you have an end draught booth, as I do, then a bit of planning when placing multiple parts in the booth is a must. Obviously, when painting you start at the upwind end and move with the airflow.

Re: Prevent dry over spray on everything?

PostPosted: Sun Sep 08, 2019 8:41 pm
by texastravis
Devilbiss Tekna, 1.3 tip TE20 cap, 25 psi at gun, default setting out of box (seemed to spray right on cardboard so didnt change it), plenty of compressor power (80 gal 16 cfm, dry air). Omni epoxy. Texas 90 degrees, slow omni reducer.

I have all the doors and hood hanging on one end and the jeep on the other. Fans are being pumped in from the side with the doors and hood. I think while spraying the doors the wind just moves the dry overspray onto the jeep. Today, instead of painting all at once, I tarped the jeep and then sprayed the doors and trim. I think that should help a lot. After spraying, there was a ton of dry color on the jeep tarp.

Is this normal? Should there be this much over spray or is my gun set up not right? The paint went on nice though. The doors and hood look pretty good I think.

Re: Prevent dry over spray on everything?

PostPosted: Sun Sep 08, 2019 8:46 pm
by texastravis
See photos. You can see the doors are upwind of the jeep. I believe this was my issue.

Re: Prevent dry over spray on everything?

PostPosted: Mon Sep 09, 2019 1:04 am
by '68 Coronet R/T
Here is some booth flow information that I gathered when building my booth. Mine does not flow anywhere near what they say is required but still keeps the booth cloud free.

To calculate the amount of exhaust air needed for the booth, a simple calculation is used.
CFM = Booth Face Area in square feet multiplied by the required flow (typically 100 FPM)
For example, and 8 X 10 filter bank (80 square feet) would require an exhaust of 8000 CFM (100 X 80) to achieve the required 100 FPM velocity.

To calculate the existing velocity knowing the exhaust volume the following formula is used:
Velocity = Exhaust in CFM divided by the booth filter area.
For example, a fan that exhausts 9000 CFM with a 10 X 10-filter bank would have a velocity of 90 FPM.

Proper airflow is not only necessary for the protection of the operator, but is necessary to achieve the desired finish quality. Flow that is too low will not draw the overspray to the filters. Much of the overspray will end up on the part as dry spray (rough finish). If the flow is too high, solvent tend to evaporate too quickly resulting in dry spray.

Re: Prevent dry over spray on everything?

PostPosted: Mon Sep 09, 2019 8:48 am
by texastravis
All the references I saw relating to the x100 for CFM calculation mentioned for fire safety. However, I could not find separate calculations for CFM for finish/over spray control.

Are hobbyists actually purchasing 10,000 CFM fans?

I am phasing out the painting and it seems to be helping quite a bit. When painting a car, should one start upwind or downwind with a cross wind application.

Re: Prevent dry over spray on everything?

PostPosted: Mon Sep 09, 2019 9:13 am
by '68 Coronet R/T
These are paint booth calculations.

I painted in a 100k professional downdraft booth once while taking a painting class.
The airflow was to the point that you could watch the over spray immediately sweep over the side and into the exhaust filters.

The owner said that you could have two panels being painted different colors in the booth at the same time and not worry about over spray.

I have about 1/3 of the airflow required by those calculations and can watch the over spray travel to the exhaust. I strove to have the entire booth air change every 2 minutes.

I couldn't afford a 10k CFM fan so I bought 4 used furnace fans that put out 1500 CFM. My booth is a positive pressure style so I am forcing fresh air into the booth and pushing the over spray out.

Re: Prevent dry over spray on everything?

PostPosted: Fri Oct 04, 2019 7:44 am
by jcmeyer5
On a positive pressure booth, should there be exhaust fans as well as intake fans? Assume for a second that I have a booth with 8000 CFM coming in I know I need more exhaust filter area than intake due to the passive nature of the exhaust, but with a fan or two (let's say, 4000 CFM), would I be creating a wind tunnel, or would I be better exhausting the overspray.

Still in the planning stage of the booth, so nothing is committed yet.