Hangar Flying => Hangar Flying => Topic started by: Bill Jacobs on April 04, 2005, 10:07:40 PM
Title: EZ Cowl Temps
Post by: Bill Jacobs on April 04, 2005, 10:07:40 PM
Hi - Just purchased a LongEZ and the paint is bubbling over the top of cylinders and exhaust pipes. Has anyone else had this problem ? Is it the paint ? If so, what is the best type to use ? Should the underside of the cowling have a heat shield ? If so, what type ? I would appreciate any help.
Bill
Title: EZ Cowl Temps
Post by: Dan Patch on April 05, 2005, 04:18:57 PM
Bill,
I've had a little problem with this over the top of the cylinders. In my case I'm pretty sure it was caused by the Feather Fill that the early VariEze's used before we figured out that this polyester based product was bad news. Anyhow, it happened a number of years after I first flew. I believe that what happened was that the paint/primer system (acrylic-enamel, Dupont Centari) became slightly porous with age, moisture migrated into the Feather Fill (polyester likes a little, ~2%?, moisture if it can get it) and then blew a couple of "steam-like" bubbles. Subsequently, I stripped the cowl down and repainted, but immediately got several bubbles back in the same random places, which I believe was caused (again) by residual Feather Fill that I failed to get off, not by hot spots on the cowl.
I've seen cowls get so hot that the paint discolored, but still no paint blisters; so I would bet that you had some paint reducer, moisture, or ??? trapped under the skin of the cured exterior paint (unless the paint is definitely discolored by heat). I've even had this problem on two spots that weren't heated at all (wing & winglet), where a blister formed from trapped air bleeding up through a pin hole or residual solvent or ??? So my experience with Cenatari and acrylic-urethane (PPG Concept) suggests that the bubble problem can exist with these systems under some circumstances.
I would speculate that it is good to be sure that everything volatile (reducers, etc.) is out of sealers and primers before the final finish coat(s) are applied; and be sure to give the finish coat LOTS of time to cure out completely before applying the heat (my mistake with the Concept).
Anyhow, to insulate known hot spots, Aircraft Spruce used to sell a ceramic fabric material called FiberFax or FiberFrax? that has low conductivity and will withstand any heat you can imagine (>2000F). It's a white, felt-like fabric that's pretty flexible. Stick it down with RTV (which is good to ~500F) and cover with thin aluminum sheet/foil for abrasion protection, if you like. I haven't done this personally on my plane, except to use it directly around the exhaust of son's 240-Z where the pipe passed 1/4" away from the starter. So far, so good. Others with direct aircraft experience can chime in here...