Ask Joe Powder December 2023: Steel doors and crime scene fingerprint powder

PPCJ’s regular columnist, Kevin Biller AKA Joe Powder, answers readers questions on all things powder coatings

Hi Joe,

Paging Joe.

Local architect here. Fan of your Joe Powder column.

Question for you. We are having a custom steel door (and surround) fabricated for our office. The door sits on 4^th Street and as such, will get a decent amount of road salt on it in the winter and of course, sun etc. in the summer. The question is – what is the best finish option? The fabricator says his default is to powdercoat. But I don’t know the details (if that is zinc rich or… ?).

We used to specify powder coating ALL the time but over the years have done so less often. The issue we see is that once the coat starts to fail (whether that is a chip… or just somehow water getting under the coating) it seems almost impossible to repair. And quite difficult to fully re-do.

So, there is part of me that wants to ask our steel guy to have this door primed and painted (like with something approximating auto body paint). And part of me that wants to ask for a zinc rich PC and cross my fingers.

But figured I would ask the expert.  Thank you for any help you can provide.

Andrew Rosenthal

Columbus, Ohio

Hi Andrew,

It’s great to hear from a local architect and these are very good questions. I recommend a two-coat system over a well-cleaned and pretreated substrate. Is this a prepainted steel door, bare steel or galvanised steel? This all makes a difference. [Andrew replied – “bare steel”].

After a proper cleaning/pretreatment step, I recommend a high-performance non-zinc powder primer, partially cured (gelled), but not fully cured. Allow the door to cool, then apply an AAMA 2604 grade polyester topcoat cured fully per the powder supplier’s technical data sheet (TDS). A little scuff sand between coats is a wise step as well. Make sure the edges are coated well.

This kind of coating system will resist the extreme 4^th Street climate for many years to come.

Let me know if you have any more questions.

Best regards,

Joe Powder

Hi Joe,

We currently are being asked to match a powder so we can powder coat steel parts that need to match an aluminium anodised hinge. So far, we have not had much success.

Do you feel I’m spinning my wheels trying to match a powder to a plating finish? I’m thinking it’s kind of like the chrome powders that are offered; they call it chrome powder, but it really doesn’t look anything like chrome plating. Would I be better off asking them to powder coat the steel parts and the hinge if they want everything to match?

Thanks in advance for your help.

James Acope

Bern, Switzerland

Hi James,

Powders can be formulated to mimic the appearance of anodised silver. It won’t be an exact match but will come pretty close. This will be a low gloss metallic effect powder and some powder manufacturers offer these as off-the-shelf standard products.

It is essential to consider the environment that the assembled unit will be exposed to. If it will be the outdoors you should consider a two-coat system. This will be comprised of a metallic basecoat followed by a low gloss clearcoat. Both powders should be outdoor durable, polyesters that comply with either Qualicoat Class 2 or Class 3. These are similar to AAMA 2604 and 2605 which are used in North America. The clearcoat provides extra durability because the metallic pigments used to create the anodized effect can oxidise in outdoor conditions. The clearcoat will seal the surface and provide significantly improved durability. If these parts are for an interior application that won’t see much wear and tear then the clearcoat won’t be necessary.

Good luck!

Sincerely,

Joe Powder

Dear Joe,

Fingerprint powder is currently applied using either fine particles of carbon with a brush, or magnetised powder with a magnetic brush. Both leave streaks on the object being dusted. Also, there is growing concern about the transfer of DNA from one area to another using these application techniques. Another problem with dusting at crime scenes is that only small areas are dusted and prints in unusual places can be missed. Could the powder technology used to apply finishes be used to apply some type of fingerprint powder at crime scenes?

Thank you for your time.

Helen Griffin

Ventura, California

Dear Helen,

I think that it is entirely possible to borrow the electrostatic techniques that we use in industrial powder coating to apply fingerprint powder. Two basic techniques are used to charge our powders. One relies upon a negative corona charge delivered at the gun tip. The fluidised powder is pneumatically conveyed through a field of ionised air and becomes negatively charged. The charged powder is then attracted to a conductive substrate. Alternately powder can be applied by frictionally charging the particles as they pass through a gun comprised of a highly dissimilar dielectric material. The powder picks up a positive charge and is deposited on a conductive substrate.

The substrate must be somewhat conductive with our polymeric based powders. We have successfully coated wood and plastics, as well as the more common metal substrates. The commercial guns that we use are somewhat large and possibly cumbersome for a delicate operation such as fingerprint dusting. This doesn’t preclude using these techniques if they can be adapted to deliver smaller quantities of powders of a different nature. A number of research groups may be willing to explore these possibilities.  If you’re interested, please contact me and I will help you sort through which group may be best in which to initiate a dialogue.

I hope that this helps you with this intriguing objective.

Best regards,

Joe Powder

Readers can submit questions to [email protected]