Chapter 25: Surface Preparation with Sandblasting


Surface preparation is vital to starting the contouring process.  When cured, our epoxy systems leave a shiny surface that must be scuffed up or roughened up to a dull finish in order to establish a good mechanical bond between the dry micro and the surface.  Hand-sanding is the time-honored way to accomplish this.  But the fiberglass is a woven cloth with peaks and valleys.  Hand-sanding tends to scuff up the peaks but not get into the valleys.  Furthermore, hand-sanding tends to cut off the peaks of the cloth, thereby weakening the structure.  The harder you press the sandpaper to get into the valleys, the more you tend to sand away the fiberglass peaks.  It's certainly a catch-22.

I've known for years that some builders have used sandblasting as a way to prepare their surfaces.  They claim it saves time, results in much better prepared surfaces, and causes no damage to the fiberglass structure.  I have a humongous compressor in the hangar (thanks to my hangarmate, Steve) and a portable suction-feed sandblaster.  So I thought I'd give it a try.  I sandblasted my winglets as test cases and was immediately impressed with the results.

The pictures below show the obvious benefits of sandblasting versus hand sanding.  The area in the lower right corner of the pictures shows the shiny surface before being sanded.  (I'm using EZ-Poxy.)   The area in the lower left corner of the pictures is hand-sanded.  It took me ONE HOUR to hand-sand that tiny area!!  Note the obvious streaking caused by not being able to sand completely into the valleys of the weave.  If it were possible for you to zoom in real close, you'd see that even this little bit of hand-sanding has cut off the peaks of the weave.  I stopped at that point because I didn't want to weaken the structure any more than that.

 

Now note the upper half of the pictures.  That is the area that has been sandblasted.  It took me all of FIVE MINUTES to sandblast that area!  Notice how the sandblasting gets into the peaks and valleys, which results in scuffing that is very evenly and completely distributed.  There was NO DAMAGE to any of the weave structure.  I finished sandblasting both sides of the winglet -- including going back over the hand-sanded area -- and was done in 20 MINUTES!

The naysayers will preach about the potential for disaster.  They'll scream about blasting a hole through and through.  I'm here to tell you that's a bunch of crap.  What are they using? 12,000 psi and rocks???  You just gotta be smart about it.  I had the compressor set on 120 psi and I used extra fine black oxide abrasive media.  I held the gun about 4 inches from the surface and used a gentle back and forth sweeping motion just as one does when spray-painting.  It was very easy and very intuitive to know when the surface was roughed up enough.  You could literally watch the surface gradually change from shiny to dull.  For purposes of testing, I grabbed a scrap piece of fiberglass and intentionally tried to blast a hole through it.  I literally had to hold the gun right up to the surface and hold it still before the fiberglass eroded enough to even begin forming a hole.  The extra fine media and the low pressure provided plenty of margin for error. 

Had I known the sandblasting was going to be this EZ, I wouldn't have peel plied the larger areas like the entire fuselage and strakes!!

Be sure to wear a high-quality particle respirator and eye protection!  It's also a good idea to wear a disposable painter's suit with a hood.  The media GOES EVERYWHERE and GETS INTO EVERYTHING!  You definitely DO NOT want to breathe in any of that oxide dust.   But let me tell you, sandblasting is well worth it.  I'm a happy camper.

 


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