Paint booth or not ??

[PaulSA]

I'm just spraying in the open and have had good results - just a few specks. I'm trying to get a decent finish - not an award-winning automobile type finish.

If you make a booth, I believe you have to take extreme care with the choice of fans to avoid potential electrical sparks from the motors.

[Glenn Patterson]

Agreed on fan choices also where the air goes. The air should be exhausted directly outside and not in the shop with waterborne and solvent paints. Exhausting in the shop risks recirculating a toxic environment and could be explosive with volatiles.

[Glenn Patterson]

We did some spraying outside early in the morning with good results. Flies and wind seems to be calmer early in the day and the sun is not hot enough to affect the paint.

We used an old furnace fan with an external motor with a belt drive for the exhaust fan. The flange was fit to a plywood filter box and we made a chipboard box duct for the short run to the back door of the shop to expel through a plywood door blank. Furnace fans typically are 1200 -1600 cfm so the air change through the booth is good with no foul air. Take the booth volume divide by cfm then that will give the air change time in minutes. There is no explosive component in the Stewarts or other waterborne paint systems. I have an old car to paint and will use the same style booth with solvent based paint with the fan motor outside the foul air flow. Some furnace fans are direct drive with the motor in the fan housing and they are not the ones as they will foul up with any type of paint that risks a fire. The calculated air change was about 1.3 minutes with good air flow velocity across the booth. The furnace fan can come from an HVAC shop as the old furnaces go to the metal recycle & have no value to them..There should not be enough paint fumes for an explosion with a proper booth set up. Having said that. If a person is spraying in a shop without a booth in a building with solvent based paint then definitely a risk of an explosion.. Anything running in a closed shop with volatiles in the air could set off an explosion.

We were very careful on our booth and air management as we live north of 49. The project required paint to done outside summer & the shop had wood heat. We heated the shop to about 90F+ then partially opened a screen window in a far door then ran the booth. We shut off the fan and closed the window between coats. Shop recovered heat fast as it was well soaked into the building and the air through the booth was warm. The air was sent outside so there was no issues with the painting inside the shop.

[alaskabearhawk]

You want to be careful with any type of fan, furnace included. Typically paint booth fans have non-sparking fan blades made of aluminum. If the fan happens to contact the steel housing there isn’t an ignition source, I.e. sparking. The furnace fan components such as the housing and squirrel cage fan are typically made of steel.

[Glenn Patterson]

Agree generally with you comments. An external belt driven furnace fan is fairly low rpm fan that last forever. The odds of a squirrel cage runner hitting the case is very slim. I replaced a gas furnace in our rental house in 2010 that was installed in the early 60's. It was all original. I replaced the furnace in our residence and that was 30+ years that had to go as it was 65% efficiency. The air change in our booth was high enough that it was a very clear environment so the LEL's would not be high enough. We had fresh air in and exhaust ducted outside. I did some reading to find that furnace fans run on between 1200-1600 cfm that put the rate travel for the air through the booth about the same as a recommended for commercial booths.

Box fans have the motor in the air stream where there is electrical exposure & a lot of exhaust fans are the same. Even if there is not an explosion risk with a motor in the air stream they will get fouled with paint and could become a fire. Not exhausting the fumes from a shop when spraying solvent based could also get interesting.

I was involved in installing an 8' dia high speed turbine fan and the planners got slack on its inspection. It literally exploded through its metal case and shards went through the outside wall. That was so violent that it bent the 7" dia shaft. The shaft and runner was replaced and it ran for years and let go. I told the planners that the shaft had to be changed but they knew better & it blew up in a week. I had a project to design and build a large paint shop for the mill. in '77. All electrical was explosion proof for safety. The painters never had a paint booth since the mill started in 1910 but with some negotiating I got the budget to put in a booth. The painters used to paint and spray in the old shop with no exhaust so some days they were high on the solvents.