*when using FAI or low nitro (5%)*
What actually damages the finish? Raw fuel? exhaust residue? will fuel proof paint still be necesary with FAI fuel?

In most cases the ray fuel containing the nitromethane and methanol damages paint. Exhaust residue is usually just castor oil which may or may not hurt it. It's always best to use fuel proof paint. Better yet use covering where possible to reduce weight.

Raw fuel can cause problems as well as exhaust residue (burned fuel).

I have seen damage to painted finishes when not allowed to cure before use. I ues Rustoleum over 21st Century Fabric. It dries in 24 hours (won't stick to your finger when touched) however it takes 2 or 3 weeks to cure (harden and attain maximum resistance). I used lacquer to finish furniture. It "dries" in less than 30 minutes and takes over 6 weeks to cure!

If the burned fuel damages the paint the raw fuel will do it faster.
It has more to do with what paint you use rather than the fuel (unless using 40% racing fuel). As bob_nj says above it is better to use covering (monocoat or ultracoat etc).
Bill

Can't say much about no nitro stuff, "FAI"...buuutt the higher the nitro the more important fuel proofing is. Rustoleum is good to just about 20% nitro. I've a couple of planes sprayed years ago with restoleum enduring a life of 20%nitro. Areas of direct raw fuel contact near the cowling have rubbed paint off slowly over the years. Raw fuel will rub it off on a cloth if rubbed like it were sand paper....just don't do it..heh heh...

Don't use krylon...never drys and sticks to everything...not fuel proof...learned that one the hard way....

Another .02

hawk