Posted for Newlan2dl@aol.com:

 Amen Casey!
 
 A grinder isn't the ideal tool to use for prepping the surface on such a thin
 lamioante.  A much better tool is a DA sander.  A DA sander (dual action) has
 a free spinning disk mounted eccentrically on a fast spinning core).  These
 come in air and electric varieties.  Chicago Pneumatic makes an excellent air
 powered one and Porter Cable makes a great electric one, (I have found Porter
 Cable to make the best sanders of any type consistently and have several in
 addition to Milwaukee).  These are not as aggressive by far as the grinders
are
 but offer better control for sanding thin laminates.
 
 But in addition to that, you want to get a soft pad to change with the
 typical hard pad that comes with most of those.  For basic surface prep
without
 fairing, I use about a 1/2" pad.  The thickness and softness allows for less
 chattering and better control along with better conforming to a curved
surface.
 When it comes to fairing, that tends to be too soft and I will switch to a
1/4"
 pad or the hard pad.  Likewise, grinding discs can be purchased for different
 degrees of surface curvature and control.  But change paper often!  If it
gets
 dull, it will start getting hot and leave brown or black marks.  The best
 prep technique is to sand with little pressure and high speed.  Glass
prepregs
 dull the paper quickly so you need to change every 3-4 minutes of sanding.
 For
 thin laminates, use 80 grit or maybe 120 grit, the roughness doesn't really
 get you any extra bonding but it does go faster.  Sometimes it is easier to
see
 when you've sanded enough by drawing a bunch of zig-zag felt tip pen lines on
 the bond area then begin sanding.  A good prep job will remove all of the pen
 marks when sanded well.  Of course, no shine anywhere on the bonding area can
 be allowed.
 
 Also of use is an orbital or vibrating sander.  These are even less
 aggressive than the DA but will still save a lot of work.
 
 And being a tool snob, I hesitate to mention that a really clever and cheap
 sander that I find very useful is the Black and Decker "Mouse" sander.  I got
 mine for about $40 I think.  It is great for corners and tight areas, but the
 price of the sanding pads is startling, even for those use to paying airplane
 dollars!  So I buy the 3M long board sanding pads by the 10 sheet sleeve and
 cut out 9 pads for the "Mouse" from one sanding sheet saving several dollars
 apiece.
 
 Anyway, try a DA and it will be a real friend.  It is also great for painting
 and prepping, however reserve the final sanding or any precise finish sanding
 to an orbital.  I think you can get DA's from Grainger but try and find a
 Poter Cable, it is more expensive but they really have sanding tools
perfected.
 
 And chage paper frequently.  The coarseness of the grit doesn't change with
 use but the sharpness does and the difference is enormous, particularly when
 fairing.  A fresh sheet f sandpaper cuts the surface, while a dull sheet
rubbs
 it making fairing a lot more difficult.  You will have a fairer surface with
a  sharp sheet of paper and it doesn't sand in the low contour as would a dull
 sheet.
 
 And Casey is also correct when he says that bonding to an uncured surface is
 superior to bonding to a cured surface.  An uncured adhesive has reactive
 sites ready to bond to vs a secondary bond to a cured surface.  It reacts
more as
 if it were laminated "wet on wet" meaning the laminates were placed on top of
 each other while wet.  The analogy is doing a continuous pour of concrete vs.
 letting it harden then pouring more on top.
 
 Dan


[Just to stick in my 2c.... I like to use air tools but I don't like them to spit oil everywhere.  Consequently, my mantra has been to buy cheap air tools, run them without lubricant, and simply replace them when they finally fail. Harbor Freight is a great source for fine performing, low cost, disposable air tools.  They have models of grinders and sanders with rear exhausts that are easily shielded (by wrapping a rag around the exhaust end) to help protect from any chance of contamination that might accompany the air supply.   I wouldn't dream of treating a nice Ingersoll-Rand tool this way, but a 19.95 die grinder made in China... no problem.  FWIW    <Marv>           ]