General Notes on Various Materials
This section summarises some
general information I learned about various materials used on the project.
Fuel Tank Sealant
Fuel tank sealant, commonly
referred to as "ProSeal" is...
RTV Silicone
RTV silicone compounds are often
used as sealants, non-structural adhesives, and gasketing in various parts of the
airplane.
Corrosive vs. Non-Corrosive
One-part RTV silicones commonly come in one of two basic
chemistries, one that is suitable for use in a metal airplane and one that
is not.
Most "standard" RTV silicone is acetoxy based, and is mildly
corrosive to aluminum and other metals during the cure process due to the
release of acetic acid. This includes most common RTV silicone
products that don't explicitly specify "non-corrosive".
Note that even products that specify "sensor safe" are not
necessarily non-corrosive.
Most "non-corrosive" RTV silicone is oxime based, and is not
corrosive to aluminum nor to other metals. Most non-corrosive RTV
products are marketed as "Ultra" (by Permatex), "Mega"
(by Versachem), "XST" (by Pro Seal), etc. But in all
cases, they make sure to explicitly specify "non-corrosive".
Other non-corrosive RTV silicone products exist that are alkoxy based.
For obvious reasons, only non-corrosive RTV should be used in a metal
airplane.
Standard-Temp vs. High-Temp
RTV silicones come in several temperature ratings.
Note that this is referring to the temperature range to which they are
suitable in service. All are still RTV, as in room temperature
vulcanization, or cure.
Standard-temp RTV is typically blue in color, and is commonly referred to
simply as "blue RTV". The max temperature range
specifications vary a bit between different manufacturers, but
conservatively, standard-temp "blue RTV" is generally suitable
for service temperatures ranging from -54°C to +204°C (-65°F to
+400°F). That means that it can safely be used anywhere aft of the
firewall.
High-temp RTV traditionally has been red in color, and is commonly
referred to simply as "red RTV". But here there's a
kink. Most red RTV products are acetoxy based and are mildly
corrosive (see discussion above). Most oxime-based non-corrosive
high-temperature RTV products are actually copper colored, not red.
It's the non-corrosive copper-colored high-temp RTV that's actually the
material of choice. Anyhow, the max temperature specifications vary
a bit between different manufacturers, but conservatively, high-temp
"copper RTV" is generally suitable for service temperatures
ranging from -54°C to +316°C (-65°F to +600°F). That means that
it can be used safely on the engine side of the firewall, and even
directly on the cylinder heads. In fact, one of its key applications
is in sealing the baffling.
Silicone Contamination and Paint Fisheye
Silicone contamination on a surface prior to painting is
known to cause fisheye problems when painted. Furthermore, such
silicone contamination can be difficult to detect, and very difficult to
remove. As such, it is best to keep any silicone well away from any
surface that will later need to be painted.
References
