I'm just finishing my Trophy Trainer and have had something happen that surprised me; the dope at all areas of the plane that have a 90 degree angle, such as the wing/fuse joint has shrunk so much it is pulling away from the balsa and forming air pockets. After reading many posts here I'm assuming that the tautening butyrate I have been using may be the culprit. If I switched to nitrate as the under coat before moving to butyrate is the problem likely taken care of?

Thanks,

Brian

I don't use nitrate and don't have the problem. But I use non-taunting butyrate from the wood up so there you are.

Sounds to me like the surface needs a little roughing up.

I use it all the time... I put on two coats and lightly sand... then one more and I am ready to cover.. then fillets..(learn to make these small) they get Brodak Primer (I like this stuff).. more sanding... lightly.. when the surface is where I want it.. I switch to Sig or Brodak.. I have used Lustercote too for the fuselage.. most of the time my coats of Nitrate are put on with a camel hair brush.. one I have had for over thirty years, a 2" .. I remember coughing up a whopping $4.95 for it.. now I start to spray... misting my first couple of coats on.. nothing heavy.. and never too much too soon.. it takes a week or more to get the dope all on..  I sand religiously been every 2nd to 3rd coat.. I am not the best finisher by for but I don't get blistering, lifting or bubbling.. it took a while to learn .. he worst is mixing and matching paint... once I have commited to a brand I use it all the way through.. Sig Nitrate.. Sig Lite Cote.. Sig colors.. Sig Clear.. I use 3806 Thinner with retarder and plasticizer in my colors.. and final clear.. I really like Brodak thinner but at $40 plus a gallon... my "dope habit" ain't that bad yet... I ran into Nick Stratis the last time out and he told me to switch to Randolph... as soon as all my paint is gone.. that is next..

Paint is thinned 50/50 when it is hot and 40/60 when it is cooler (less thinner).. I have found that thinner come out of the gun kind of cool and if temps and humidity is at the limit blushing can occur.. if it is 68-88 degrees and rain is not in the forecast... start painting..

Jim

From what I've read on these forums, nitrate has more solids it fills the balso grain better thus making a better base coat than butyrate. Since it doesn't tauten as it drys I should gain two benefits by using nitrate in the beginning.

My plan is to use nitrate as the base, then tautnening butyrate over the open bays and rustoleum as the finish coats. I think this should work, based on what I've read on the forums. I'm also going to look into an air brush for future planes; the rattle cans just make it so much easier than brushing.

Brian

Hi Brian,

This is a pretty late reply but I don't get to this part of the forum very often.  Nitrate comes in both tautening and non-tautening type.  Tautening nitrate vs. tautening butyrate shrinks much more (in terms of inches per inch of length) than tautening butyrate.  However, nitrate film as applied has a lower tensile strength than butyrate by a fair amount.  Also, all dopes shrink whether they are tautening or non-tautening.  Tautening types shrink approximately 10-12% whereas non-tautening types shrink about 5-6%.  You can add a little plasticizer such as Dave Brown's "Flex-all" to the dope (tautening or non-tautening), but be careful with it because if you overdo it you will begin to have intercoat adhesion problems.

If you are going to use nitrate as your base coat (and I don't see any reason to use nitrate except for full-scale airplanes as required by the STC for the covering) then use the non-tautening type and don't put much in the intersections of the wing/fuselage etc.

Again, your comment "Since it doesn't tauten as it dries..." is incorrect.  Nitrate (and butyrate) does tauten even if it is the non-tautening type.  That is why it is pulling away from the perpendicular intersections.  The nitrate just shrinks so much that nothing is going keep it stuck down.