Hi Gary et. al.

Intumescent coatings are great...sort of.  They really really work BUT they
are extremely fragile coatings once they intumesce, i.e. expand to a highly
insulating ash.  These coatings which expand thousnds of times their original
thickness can protect a surface from thousnads of degrees but they also can
generally be blown away with very little wind.  Remember those old fireworks
called "snakes" where you lit the pellet and it expanded to this long winding
string but you could blow it away with your breath?  That is like an
intumescent coating.  So if it will be in the slip stream or cooling air,
unless the prop isn't turning, it's pretty useless.

As to brominated fire retardants they can add a lot to a flame resistant
coating also but they tend to act like a plasticizer on resin reducing their
bonding, strength and modulus.  And it also tends to reduce water resistance.
 Probably not a big deal in planes unless some standing water puddles but it
drove one company to bankruptcy, (Valiant Yachts) when they were really
trying to do a wonderful thing, make boats more fire resistant.  They are
back in business and make a fine product (kind of slow for my tastes which
run to racing yachts but they are well built, comfortable and safe).  Anyway,
their brominated resins allowed the hulls to absorb water and soon had
blisters all over the bottoms of their boats.  The only solution was to
completely strip the gelcoat off of the bottoms of the boats and completely
cover the bottom with layers of glass and epoxy then fair and repaint the
whole thing!  And you complain about sanding and priming your planes, try
several times the area and doing it upside down!

Dan Newland

Super ES