B maniac

I believe you've said previously that your booth installation is fairly recent. Do you recall having a similar problem when you were just painting out in your open shop area - before the booth? Just trying to get a grip on whether this is a problem that likely has nothing at all to do with the booth.

Have you ever tried painting parts in your booth with the booth doors wide open? I think it is technically possible that you are aggravating the dust problem by closing the doors. I know that sounds silly, but bear with me.

I understand that you have a walk-in booth for painting parts that is located inside a larger shop area. You have fan(s) that exhaust from the booth at the end opposite the doors, creating a negative pressure inside the booth relative to the surrounding shop. You have doors to the booth that total 64 sf. I'll assume that the cross section of your booth is also at least 64 sf. You have about 18 sf of filters in the doors. Your ratio of filter area to booth section area is 18/64 = 0.28; a little low but not unreasonably so. The standard air flow design, as you know, is 100fpm across the booth section. That's 6400 cfm for your booth. If you are running your fans slow to get rid of overspray using a minimum air flow, let's assume you are running at half the standard design or 50 fpm = 3200 cfm. At that flow, the face velocity at the filters is about 175 fpm (3200/18 = 178). Any crevices or openings that may exist to permit shop air to bypass the filters are going to have an air velocity through them that approaches this number. Any clearance gap between the booth doors and the shop floor is particularly important since shop floors are notoriously dirty/dusty. With the doors closed and an obvious gap at the door bottoms, the effect is not unlike a vacuum cleaner sucking up dust off the floor and exhausting it directly into your booth. It can also stir up dust on the floor just inside the doors and help to spread it into the general airflow pattern.

I know you haven't concluded that your problem is caused by dirt, or solvent pop, or something else. However it might be worth a quick "feel test" with your hand at the door bottom clearance (assuming there is one) with the doors closed and the fans running at the speed you normally use. Just to get an idea as to the airflow at that specific location. You could even do some test painting with the doors open. With the doors open, the face velocity at the booth opening would then be 3200/64 = 50 fpm on average, even down near the floor level. That lower velocity, being a third as great as with the doors closed, would be less likely to stir up dust and carry it into your painting area. Unless your shop area has a lot of normal airborne dust anyway, you could be better of with the doors open than with them shut. Perhaps not probable but certainly possible. Wetting the floor down just outside (and inside) the booth doors or installing door weatherstripping at the bottom would help. It's good to seal up as many leak areas with duct tape or caulking compound that are feasible, but I think the most important leak area is at the door bottom clearance and should be given priority in any sealing efforts. I think this is the case even if you find out your present problem is not related to dust at all.

Food for thought only. Discard as desired.

Rod

Rod

I see I was writing when you were posting to Dave above. If I had known you were responding, I wouldn't have answered since your post was correct and didn't need anything added.

I have never actually measured any intercoat flash time. I don't think it is that critical either unless you are way out of line from what is recommended. For MTK it's 5-10 minutes at 70F per the tech sheet. At the temperatures I paint, I'm usually not far above that. I just stop for a while, go stir the remaining paint, assess whether I have enough mixed to complete the parts(s) I am doing, go in the house, have a drink, come back and get onto the next coat. I don't know if it's been 8 min or 15 min and it varies each time, simply because of the random nature of my doing it. I am pretty sure it is always significantly longer than 5 minutes though. If I'm at 80 or above, I might make a "mental adjustment" to wait a "little longer" but it's not any kind of calculated or measured process. At 75 though, I wouldn't make any mental adjustment at all - it wouldn't even occur to me to bother.

If you have good flowout with the 167, there is no reason for you too change it that I can see, even if you are outside my arbitrary temperature ranges. If you really suspect that flash times are the cause of your problem, I'd just make sure that you give it at least 10 min if you're near 70 and consciously extend it if you're below. Say, maybe give it 20 min if you at 60. I don't know, but think it's more important to lengthen the time when you are cooler than normal than to shorten it when you are warmer. It's also probably important to lengthen it when doing small pieces. Size is not accounted for at all in the tech sheets, but should be a factor, in my way of thinking. When you're spraying a whole car, there is some amount of automatic built-in flash time just because it takes a while to complete a coat and get back to where you started. If you're painting a small part, that transit time is essentially zero. DuPont's recommendation for Centari is no flash time at all between coats - has been for the last 25 years (contrasted with Delstar's 15-20 min). I've given Centari some time when I've used it more recently, simply because I was painting a panel at a time, not an overall.

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