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Need Advice: Finishing Aircraft Polyurethane

Posted By:
John Jacobi
Homebuilder or Craftsman
2
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#1 Posted: 9/13/2009 10:32:35

No matter what I do, I can't get a perfect finish over a whole part.  I either get a bit of a run, or more likely some kind of orange peel.  I can adjust to succesfully spray on a vertical piece of tin foil but *something* changes during the spray that has me re-sanding and re-painting for almost 18 months now.  To get around this phenomena, I'm now looking at fine sanding and polishing out these relatively small defects to a fine finish.  The suppliers give me some generic advice but nobody has specific instructions for a fabric covered aircraft with rib lacing, hand stitching, pinked tape edges, etc.  Does anybody have a formula for success polishing out aircraft polyurethane on a fabric covered aircraft?



Lee Hudson
Homebuilder or CraftsmanAirVenture Volunteer
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#2 Posted: 9/17/2009 12:14:01

Your are probably not going to have much luck polishing poly

You are probably not going to have good results polishing polyurethane over fabric.  It cures very hard and will require

too much pressure on the surface to polish.  You may be able to polish within 24 to 36 hours after spraying, but as it continues

to cure it only gets harder.

You didn't mention what paint you are using, but I think you are probably going to need to talk to someone with some

experience with spraying and maybe upgrade to better equipment for the job.  Many of the new hvlp spray guns are difficult

to get to work properly and do not provide atomization fine enough to avoid oringe peel.



Wayne Snider
Homebuilder or Craftsman
1
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#3 Posted: 9/22/2009 06:08:00

Hi John, 

Several years ago I painted a Baby Ace that I spent 4 long years rebuilding.  I used Dupont Imron over both the fabric and the metal.  While sitting in a local hangar, some idiot decided to spray his airplane (in the hangar). My plane and four others were covered with white overspray.  Long story short, we all sued the owner of the  company and he fled the state.  The other planes were insured, mine wasn't. I could not bring myself to recover it, so I decided to polish the overspray out (by hand) with three different 3M compounds. It took many long hours of work, but in the end it worked. Sanding near rib stiching is of course, a fatal error. The Imron polished out well.  Be careful if the paint is thin, as mine was in a couple areas. Any body shop will have the different grits to use.  Use only soft, well worn towels (I buy lots of them at Goodwill). You will go through many, many towels, as you have to constantly keep a clean surface to give some bite to the compound.Don't use a power buffer on any fabric....instant disaster awaits if you make one slip.

 

Wayne



John Jacobi
Homebuilder or Craftsman
2
Posts
0
#4 Posted: 9/29/2009 22:08:50

Thanks both Lee and Wayne for the input.  I've got a fairly expensive rig known for superior atomization, same as used by a professional nearby, and as I mentioned I test for adjustments and spray my final test patch on vertical tin foil, but something always seems to change during the spray.  Perhaps the temp of my input air heats up and volatilizes some of the solvent or accelarates some of the curing and voila, the last 1/3 of the spray session is slightly imperfect, often not showing up until 15 min. after I finish.  Or, when I mask and re-paint a panel, the step-off is unavoidable.   I'm painting at 5000' altitude in the dry, and while adding thinner solves the texture problem, it makes it difficult to avoid accumulation when transiting an edge.  At some point I just have to stop sanding and re-painting.

I'm having some good luck sweeping (hardly touching) with 2000 grit, followed by a fine cutting compound (#2) followed by a finishing polish.  I also have a slow speed polisher that removes any small scratches left by the paper.  The resulting surface still beads water same as the original, but I get a much smoother feel.  I'm using Ranthane and it is very flexible, so it takes a lot of focus and attention to blend in a step-off or tape ridge.  I've used Imron on the frame; it is great on that, but the flexibility and structural cohesiveness of Ranthane is astounding.  If only I could get it to flow with one setting in these conditions.

Wayne - do you recall the 3M compounds you used?  Were they machine products applied by hand?  I'm not trusting anything yet that isn't mainstream in the professional finishing business.  Thanks again.