We're restoring some old furniture - sofas with a lot of carving, whole wood book shelves, et al. We've stripped the old varnish and are redoing the job with poly. We did 2 coats of PU sealer and 2 coats of PU Matt, all with a spray gun. Most of the wood's teak stained with walnut, some of it is rose wood, so we're getting a nice dark brown.

For some reason, when spraying the top coat say onto one side of say the sofa's arms / legs or a shelf's side, the other finished sides pick up rough grey patches. The contractor reckons it's air from the spray gun that's causing that and says it's inevitable when using matt PU. We'll be trying again by covering the finished sides with paper and painter's tape, but we can't do this with the sofa's arms / legs as easily.

We also tried wet sanding with 1500 sandpaper - which removed the roughness, but left the grey patches - followed with 3m finesse it paste wax; it's too shiny for our liking and it raises the wood's grain, the grey patches / new scratches are now visible.

This is the brand of PU we're using: https://www.bergerpaints.com/products/wood-finishes/56/imperia-luxury-polyurethane

Is this normal? If so how do we avoid these rough grey patches in the first place?

Bed frame Bed frame's leg Bookshelf's side

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    My first thought was overspray but I think looking at it more carefully there's more going on here. And there are clear indications in a few areas that stripping wasn't thorough enough to remove all traces of the original finish. – Graphus Sep 14 at 6:34
  • One tip you may appreciate for future projects, often "sealers" are just diluted finish, which of course you can make yourself. Unless you're using a specific sealer product for a specific purpose (e.g. vinyl sealer or shellac) the first coat of any finish acts as the 'sealer' coat. – Graphus Sep 14 at 6:39
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    There's lots the finish manufacturers don't tell us! The entire industry is full of proprietary terminology, sometimes for marketing reasons (just copyrighters or execs getting their few words in) and sometimes to deliberately obfuscate (to hide the basic simplicity of many products such as "wood conditioner" and "sealer"). Back to your piece in next Comment. – Graphus Sep 14 at 13:19
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    I think I see here and there clear signs of remains of the original finish, so at least some of the appearance issues I think relate to that. Regarding the overspray, you will 99% have had some as it's nearly impossible to prevent with normal spraying (i.e. not using HVLP) so you try to be aware of where/how it occurs and work around it e.g. by spraying the surface that overspray has landed on next, before drying. I can't cover overspray in sufficient detail to be really helpful here (whole chapters are devoted to solving individual spraying issues) so you will need to look up more on this. – Graphus Sep 14 at 13:28
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    @FreeMan, stupid fingers! Meant copywriters <doh> – Graphus Sep 14 at 18:26

The spray gun is generating an electrostatic charge in the spray and atomized particles are attaching to the nearby surfaces of the wood. It looks white because the varnish has been atomized (like a white mist).

There are a few things you can do

  • cover the other areas
  • increase the humidity in the room (not recommended for woodworking)
  • attach a grounding wire to the spray gun
  • place a sacrificial piece of grounded copper pipe near the wood
  • use a larger particle size for the spray nozzle
  • run fans to change the air in the room with outside air before spraying

The lack of humidity is the main problem here. The dry air makes for low electrical resistance and charged particles travel further to negatively charged surfaces.

  • Nice Answer, but aren't you assuming the low RH here? IF the OP is where I think they are low humidity wouldn't be a problem :-) – Graphus Oct 26 at 4:50
  • @Graphus humidity reduces the distance a particle will travel when charged but doesn't stop it. – cgTag Oct 26 at 14:18
  • Yes sorry my point was aren't you assuming the OP's main problem is low humidity? That's how I'm reading your closing sentence. – Graphus Oct 26 at 15:22

... how do we avoid these rough grey patches in the first place?

I can’t think of any reason why this would be coming from within the wood itself. Whatever it is, overspray, dust, etc, it’s probably coming from the air.

So to avoid it, try covering the surfaces you aren’t working on with plastic sheeting. You can stick it down with masking tape, or clamp it under a piece of scrap, or whatever you have that keeps it in place without damaging the piece (e.g. for legs throw a garbage bag over it and wrap some tape tightly around the top of the bag, whatever, be creative).

You could also try tweaking ventilation and airflow if it’s overspray, e.g. a fan facing you might push the overspray at you instead of the piece. If you’re in a room with little/no airflow, then where things land is up to gravity, static, and the whims of closed air currents.

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