RE: whats right for the kougar
Aileron flutter can be caused by several things. First make sure there is no play in the linkage. The leading edge of the aileron should be as thick or thicker then the trailing edge of the wing where it mates. If it isn't or there is too much bevel from sanding you may need to do more then just seal. On my Shrike I used MonoKote and CA to secure a strip of 1/64 ply to the trailing edge of the wing and had it lay over the aileron. This got rid of flutter when everything else failed.
I had a Kougar in 1980. It is a 40 size, right? I would drop down to a 9x8 or 9x9 prop.