|
Letter 15044
Outgasing on powdercoated sandblasted
steel [Indiana]
++
I just opened a small custom powder coating shop. We are coating a
lot of used steel patio furniture. We sand blast the items and then
coat them. We are having a problem with outgasing on heavily rusted
areas. We are learning daily and want to put out a good finished
product.
Any and all suggestions would be greatly appreciated.
David Willibey
- Bloomington, IN
++
David,
I assume that you have sandblasted the rust away completely.
Powdercoating over rust or oxide is "never" a viable alternative,
because the rust will continue to grow, and eventually will peel your
topcoat. Also, if you have not completely removed all of the rust,
you can have topcoat performance issues such as the ones you are
having. You may want to try changing the abrasive media and/or
coarsness of the media. Sandblasting can usually work well in this
type of situation, however, care needs to be given to even media
distibution and completeness of the area needing to be cleaned.
Remember that the type of media and it's size will determine how the
surface profile will look after the topcoat has been applied and
cured. You may want to test different media abrasives on test parts
first. Also, make sure that you have properly pretreated the metal
soon after you sand blast so that it doesn't start to rust. (At this
time, there is NO organic protection from the element! /s so rust
comes fast). It may take a bit more impingment pressure if you are
using large media. Make sure no organics are left on the part (in the
valleys) before topcoat or they may well offgas during the cure
cycle.
|

Bob Utech
- Brooklyn Park, MN
|
Editor's
note:
Mr. Utech is
the author of --
|
|
++
Thank-you for the response. I am wondering about the chemical
reaction that creates the gas. It must be activated by the heat of
the cure oven. Would a chemical bath, (pickeling ?) neutralize the
corrosion? We deal with thin and very old substrates that are
destroyed by the heavy forces of mechanical cleaning.
David Willibey
- Bloomington, IN
++
There need not be any CHEMICAL reaction. The rust is inherently
porous and thus is full of air. When the component is heated to cure
the coating, that air, now trapped under the coating, simply expands
as any gas expands with increased temperature. Either get rid of the
rust beforehand, perhaps by use of an inhibited acid pickle (which is
likely covered in the archives of this site), or else impregnate the
rust with some sort of resin so that the cavities in the rust are
filled and thus can't store air. But the latter course could well
require vacuum treatment to get good impregnation and of course the
filler would have to remain inert at the coating curing temperature.
Bill Reynolds
- Melbourne, Victoria, Australia
++
Thank you for the response. I am still confused. I am sand
blasting these parts till they free of rust(I think). The parts are
not white metal but they are free of rust. Some finished parts look
great and others look like a sponge. There does not appear to be any
rhyme or reason to what turns out good and what doesn't.
David Willibey
- Bloomington, IN
++
David,
We have been operating a small custom coating shop for a couple of
years now. We do a lot of auto streetrod and restoration stuff. We
have not had the problem that you describe but you may find this
interesting. We do a lot of aluminum parts, and they outgas like
crazy. What we do to prevent this is to sandblast the parts with
aluminum oxide media (120 grit) then prebake the parts at 250 to 300
deg F for about 20 min to 1/2 hr. Then let cool to room temp and coat
and bake the coating. The prebake seems to get rid of the gas that is
trapped in the casting. This may work on your rusty steel. It's more
expensive to do from a time and power point of view but it works for
us. It is possible that the media that you are using is too coarse,
this will close up the smaller pores and trap some rust inside that
will not be clean, even though the parts look clean. Try a finer
media. Do not use glass beads, doing so will really close up any
minute porosity and give you nothing but grief.
Larry Catillo
- Thunder Bay, Ontario, Canada
++++
How well does steel and aluminum pre-treated by blasting with
Bicarbonate of Sodium (soda blasting) respond to powder-coating?
Thanks...
Ron Reaves
soda blasting - Memphis, TN, USA
++++
Dave
Can you tell me if the rusting is occuring where there is a metal
to metal contact. If so I would suspect that the powder coating
cannot get into these areas which would cause this problem. As for
the outgassing problem with casting: If you preheat the part to a
higher temp then what you cure at say preheat to 400 deg then cure at
375 deg this should solve your problem.
Jim Rice
powder coating shop - Brownsburg, Indiana, USA
Dear Reader: please choose what you want to do.
- I want to post a
question or inquiry of my
own.
-
- I want to answer or follow-up on this question
publicly (in non-commercial
fashion).
-
- I am a supporting advertiser and want to
reach
the inquirer privately.
 |