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Paint blistering on cowling from heat below the engine

Arlen

Well Known Member
When I bought my -6, there was an area below Cylinder #2 where the paint had blistered off. Presuming it was from excessive heat in that area, a heat shield was laid inside the bottom cowl in that location and the glass and paint were repaired.

I believe it's a standard -6 cowling, and there's a Lycoming O-360 inside. The paint used was PPG Concept polyurethane. There do not seem to be any exhaust leaks.

In about 25 hours of flight since the repairs and re-painting, I now see the beginning of some tiny paint blisters again. Most of them are in a line around what corresponds to the outside edge of where the heat shield sits inside the cowl, but a couple are "through" the heat shield.

Is this a known issue with RV cowlings? Is there a fix?
 
Make the clearance

I posted about this several years ago but I am too dumb to provide you with a link to the thread.

According to Larry Vetterman, Van's made a running change to the cowl that resulted in a tight clearance to the cowl. He modified his exhaust system for more clearance but I didn't want to spend the $$$$ for an entirely new exhaust system.

Here's what I did:

1. Loosen exhaust system fasteners so you can drop #2 exhaust pipe from the stub pipe that bolts to the cylinder.

2. Cut off about 3/16 - 1/4" of that stub pipe.

3. Reassemble exhaust system.

4. drill a new hole in the "dogbone" for the #2 joint attachment.

You will now have enough clearance to install one of the Van's heat shields. I riveted a piece of high temp (orange) silicone rubber onto the heat shield. Use flush type rivets and countersink so rivets are slightly below the surface. This will protect the lower cowl if there is contact (probably is) during the "wet dog shake" engine start-up and shut-down. I also put the Van's reflective aluminum foil heat shield on the lower cowl.

I have flown the pane for 100+ hrs. in conditions that included 95 degree ambient temperatures and long climb-outs. The repainting on the cowl has not blistered in that time.

Larry Tompkins
544WB -6A
W52 Battle Ground WA
 
You may also look for contamination in the fiberglass. I have the same problem and attributed it to heat also. However in seeking an answer to prepping the surface for repaint I found that this is a common problem on Corvettes. (fiberglass bodies) The surface has to be completely cleaned and the contaminant removed or the pain will blister again. Something you may want to look at.
 
Bump.

Some nice feedback so far, but wondering if anyone has any other ideas....before my friend and I look it over and discuss options this weekend.

Thanks!
 
Add a heat barrier to the inside of the bottom cowl:

IMG_1209.JPG


IMG_1212.JPG
 
Fiberfrax and heavy-duty foil bonded with RTV makes an excellent cowl insulator. Keep the Fiberfrax about 1" inside the foil border to ensure a good seal.
 
Fiberfrax and heavy-duty foil bonded with RTV makes an excellent cowl insulator. Keep the Fiberfrax about 1" inside the foil border to ensure a good seal.

Right on...a reflector over an insulator.

Fiberfrax felt works well under the Vans self-stick reflective aluminum foil.

2uo5dzp.jpg
 
Nice work, Bill. Does that foil come adhesive-backed? If not, what glue do you use?

Yes, it has an adhesive back. You can order it from Van's.

That said, I like the two posts above, even though I haven't had any problems.

One other tip, before applying your heat barrier, sand the inside of your cowl and seal it. In my case I put on three coats of 50/50 epoxy and acetone with a 3" high nap paint roller, letting each coat harden for 24 hours and then let it sit for a week. I then gave it a light sanding before putting the foil on. The epoxy/acetone will have the consistency of water and will run in and fill the holes. You could use 100% epoxy, I just elected to thin mine.
 
Foiled again

I had the Van's aluminum foil which is adhesive back applied to the bottom fthe cowl. It came up on the right side so I put some contact cement on the cowl and the aluminum to tame it a little. After about 30 hours, it came up again on the right side. I am not seeing any damage to the James cowl. Engine in an RV6A with an IO360M1B. I have the cabin heat on the right forward cylinder exhaust.
Any advice?
 
If your cowl has any oil or other similar fluids on the surface, the foil will not stick very well. Ideally one would put the foil (I used 2" wide adhesive foil tape) on the cowl before it has an engine run inside it. Otherwise, clean very carefully and consider adding a layer of thinned epoxy before the tape.

YMMV

Greg
 
Somewhere on this site a tip was posted to paint epoxy over the edges of the foil after installation. The intent is to seal it to prevent oil from migrating under and ruining the "sticky bits".

I did and it remains OK although I can't say if the epoxy is the reason or just the initial install on a clean sealed cowl.
 
Does the inside of the cowl have a painted finish, just like the outside? Vans foil sticks very well to a clean, painted surface.
 
I've used aluminum foil and contact spray. It'll hold for a long time (estimate several hundred hours even though mine never came loose after 700-800 hrs.) Place the shiny side up (toward the engine).
 
Next steps

I cleaned the inside of the cowl with Naphtha and then Acetone, sanded it and then coated the inside with a thin layer of thinned epoxy (thanks to Joe for the Suggestion on the thinned epoxy). Tomorrow I will look at the surface. If good, I will sand, clean with lacquer thinner and then lay the aluminum foil down in strips cut from Van's sheet. As suggested about, I may coat the edges with epoxy.

I have a plenum and someone asked if Aluminum foil is need above the engine. After 40 hours, the plenum cover is not having problems.
 
Does the inside of the cowl have a painted finish, just like the outside? Vans foil sticks very well to a clean, painted surface.

Dan,
If you were asking this in response to my post, then yes I used the cowl interiors as a place to put leftover epoxy from other jobs. This gave me opportunity to create a smooth inside cowl surface.
 
better product than the Van's stick-on aluminum

http://www.koolmat.com/zc.htm

P/N 04248

Ths stuff (at the top of the page) looks like commercially made version of what DanH says to do with the aluminum over Fiberfrax, but a bit better in that is has peel-n-stick over the whole sfc. Edge treatment would be required after installation - I think the tape sealiing method would work fine, tho the instructions say to use RTV - they sell the high heat version of that product if you need it (same as what you get from ACS).

The aluminum cover on this product is also quite a bit thicker than the stuff from Vans, so have a good pair of scissors on hand.

I like the thinned epoxy treatment for smoothing the inside of the cowl too. If possible, heating the cowl to 120F for an hour or so will speed up the epoxy cure. In Texas, we use the sun for such tasks :).

The stuff at the bottom of the page would be used in an interior application, and for some reason, the peel-n-stick is not very strong, so an additional glue must be used.

Koolmat aslo makes insulated exh covers, but those seem to be VERY expensive. If a simple shield won't do the trick, their insulated shields would seem to be the next step.

Carry on!
Mark
 
Does Foil Sheeting Really Work

Does applying this foil to the underside of the cowl keep the cowl fiberglass cooler? Really?

On one hand it seems like it would reflect heat away from the cowl fiberglass. But then it seems like engineering thermodynamics says the heat inside the engine compartment is going to radiate thru the cowl until it is all one temperature and foil would make no difference. There is no place to reflect the heat. It is just going to pool up in the engine compartment and the whole compartment and cowl will become one temperature thru out. I could believe a 1/2 thick insulation blanket would work, but does reflective foil work? Does it reduce heat transferred to the fiberglass and associated cracking of fiberglass? Really?

thanks
 
Does applying this foil to the underside of the cowl keep the cowl fiberglass cooler? Really?

but does reflective foil work?
thanks

I think a refresher in heat transfer is in order. Yes it helps - like an umbrella shades you in the sun.

Heat transfer = radiation + conduction + convection. The view factor of radiation dominates when close to the pipe and reflection helps a lot, a radiation barrier would be better if there is some air gap on both sides. In essence, the fiberfrax serves part of that function.

You are right there is some conduction (but mostly convection), but that is from the bulk air temps. The temperature rise of the air across the heads is not very high. Check DanH's data on under cowl temps.
 
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Vans foil enough, or is Fiberfrax needed too ?

Right on...a reflector over an insulator.

Fiberfrax felt works well under the Vans self-stick reflective aluminum foil.

2uo5dzp.jpg


I have some light charring of the unprotected white primer inside the cowl of my RV6 at ~50hrs TT (and unfortunately, some blistering on the outside too). I have sanded and repainted the inside and will be applying the Vans aluminum foil, sealing the edges with epoxy.

What is the consensus on Fiberfrax ? I understand the theoretical advantage of additional thermal resistance, but is this belt AND suspenders, or does the Aluminum foil need some help to properly do the job ? What have those using just foil experienced on long term use? If Fiberfrax is used, what is the best method to adhere it to the painted inner surface of the cowl ?

Thanks,
Peter
 
Peter,
The fiberfrax in the photo is not adhered in any way. It's simply trapped under the self adhesive foil. The foil is not edge sealed. If you finish and paint the inside of the cowl just like the outside, the foil sticks very well. Most foil release problems are due to a poor surface.

As for " fiberfrax or no fiberfrax", a simple test will tell you more than opinion. Stick foil and foil-fiberfrax sandwich samples to the cowl, point a heat lamp at them, a compare the opposite sides with an infrared heat gun.
 
Does applying this foil to the underside of the cowl keep the cowl fiberglass cooler? Really?

On one hand it seems like it would reflect heat away from the cowl fiberglass. But then it seems like engineering thermodynamics says the heat inside the engine compartment is going to radiate thru the cowl until it is all one temperature and foil would make no difference. There is no place to reflect the heat. It is just going to pool up in the engine compartment and the whole compartment and cowl will become one temperature thru out. I could believe a 1/2 thick insulation blanket would work, but does reflective foil work? Does it reduce heat transferred to the fiberglass and associated cracking of fiberglass? Really?

thanks

Not an Engineer, but it works. The ambient temps under the cowl are probably < 200*, which is no problem for the cowl/paint. The exhaust pipes are probably 800*. It is the heat radiated from those pipes that you want to reflect. If the exh pipe is 800 and the ambient air 200, something 1" away from the pipe will be much hotter than something 6" away, due to that radiated heat reducing over distance as it is absorbed by the ambient air. The goal is to reflect the radiated heat and let it get absorbed into the ambient air instead of absorbed into the cowl.

Larry
 
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Peter,
The fiberfrax in the photo is not adhered in any way. It's simply trapped under the self adhesive foil. The foil is not edge sealed. If you finish and paint the inside of the cowl just like the outside, the foil sticks very well. Most foil release problems are due to a poor surface.

As for " fiberfrax or no fiberfrax", a simple test will tell you more than opinion. Stick foil and foil-fiberfrax sandwich samples to the cowl, point a heat lamp at them, a compare the opposite sides with an infrared heat gun.

As usual, you're spot on. Even a piece of 8oz fiberglass cloth (not bonded or anything, just dry cloth) under the foil makes a noticable difference.
 
FiberFrax

Question on the Fiberfrax. What thickness did you use? 1/16" or 1/8"?

I used the 1/8?. McMaster-Carr had the best pricing I could find. I used the Vans foil over the top as described above, then sealed the edges with epoxy.

Has been perfect so far - I suspect the 1/16? would be adequate, but I didn?t want to do the job twice. I don?t believe there is any meaningful difference in weight or ease of installation.

Peter
 
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