Model Forum / Radio Controlled / Air Models / September 2003

you might try prepping the surface with a fine grain sandpaper and/or
fuel proof primer before you paint.

 Perhaps you are looking at the problem from the wrong direction.
 It makes no difference how fuel proof (or not) the paint is if it
can't adhere to the substrate :-(
 Ensure that the cowl is completely clean, gently abraded to provide a
good key for a suitable primer and then properly painted.
 
(i.e wiping it down to remove all loose paint then respraying) will just
result in a repeat of the problem.

Which cowl do you have?  Fiberglas and styrene take different approaches.
Even though many kits say they have ABS, they usually have styrene.  ABS is
very flexible and hard to crack.  Styrene is more rigid and cracks much
easier.

The best primers are automotive filler/primers.  These are very light and
contain a lot of solids so they fill small scratches and holes very well
without adding significant weight.  Most automotive primers are a lacquer
base and just about any other finish can be applied over it.  I have applied
acylics, dopes and polyurethanes over lacquer primers with no problems.

With fiberglas, lightly wet sand the surface with 320 or 400 sandpaper to
remove the gloss.  Next, you need to use a prep wash that will remove all
traces of mold release agents.  There are commercial washes for this but
liquid dish soap usually works just as well.  When spraying the first coat
of primer on, look for pinholes and other blemishes.  These usually show up
as rings where no primer sticks.  Fill these with automotive glazing
compound and sand smooth.  After you get the first defect-free primer coat
on, wet sand it with 400 until you can just stat to see the glass
underneath.  Clean and dry it and put on one more thin coat and wet sand
that only to a smooth surface.  Now you are ready for paint.

For Styrene and ABS, you need to remove the surface plasticizers.  Denatured
alcohol is good for this.  Then, lightly sand with 320-400 to remove the
gloss.  Wet sanding is preferred as it prevents the paper from loading up
and rubbing the powder into the surface.  Do the primer just like the
fiberglas cowel.

If you are going to use a light or transparent color, use the light grey or
white primer.  If you are using a dark color, use a dark primer.  If you are
using  both dark and light colors, use a light primer and apply the light
color first and then the dark color over that.  Otherwise the dark color can
show through the light color.