Blistered paint on M5-235C cowling

[TomD]

My last paint job on the cowling was blistered and during winter repairs I had it stripped and painted again.

Flew it twice in Pacific NW weather ( max temp 70F ) and it is blistering again.

The shop that did the job is trying to sell me on the idea that the dark green paint on the top is the culprit but I am not buying it. We measured the temp aft of the engine on the boot cowl and the temps between the yellow and the top green was only 15F in full noon sun.

I remember we put new baffle gaskets in a few years ago and when I put my hand on the cowl I can tell the temp below the baffle line is far below that above it.

Opening the oil door and parking it into the wind may help somewhat but I am thinking this is going to take something a bit more aggressive to solve.

Any solutions out there? Jeremy, Dave, anyone??

TD

[a64pilot]

Sounds like you have an IR thermometer. After flight, while it heat soaks, check and see how hot it gets. It may not be the paint, it sounds more likely to me that it's the primer, body putty or some other underlying layer that didn't get stripped. The paint manufacturer should be able to tell you what amount of heat the paint can take.
I could see how color would affect how hot it got if the paint were exposed to the source of the heat, but since it is on the outside, I don't think it would matter.

[donknee]

The top of the cowl on my Maule is black. I don't have any blistering problem (its over 30 years old and shows it!) Static in 85 degree temps I get 20 degree increase from black to white, so black was 105 and white 85. I would have to concur with A64. Polyester fillers can "outgas" solvents, and if painted over before a full cure, the paint will trap the gas into a blister. I tend to like to use epoxies and then use microballoons, fibres, carbon fibre or kevlar, glas cloth, etc. It doesn't outgas like the polyester products. There are other problems with contamination of the surface prior to painting that effect adhesion, which with temperature differentials can blister. I would think that a good epoxy primer would seal the surface good enought to apply most any paint product.

[Ian]

What kind of paint was used. I restored my cowl over a year ago by sanding off old paint (no chemical strippers) then patched a couple of small holes with glass roving and polyester resin. I also replaced all anchor nuts and dzuz fasteners with stainless . I then primed with high build primer and top coated with UTech 3.5 in Dune white. Utech is what Maule uses now it is a two part paint that cures with significant flex . No blistering.

[m-4-235]

Hi TomD,
I had a restoration shop for several years and I've had this problem myself. If they used any chemical strippers not made for fiberglass or used a prep solvant (any form of a wax and grease remover) than this could be your problem. The chemicals find there way into the fiberglass and stay for a long time. Now lets paint it and seal in the chemicals, add some high heat and start to boil it off. The gasses start to escape and now you have bubbles and blisters ! This may not be your problem , but I've been down this road.

Dan

[maules.com]

M4-235, is that M4 the one from FL owned by the original author of the Maule Newsletter, back in the dark ages before computers?

[m-4-235]

Hi Jeremy, If that was John Colman from FL than yes it is .

[maules.com]

That was the fellow, wonder where he is now?
The plane was quite the performer in the mid/late '70s

[m-4-235]

Hi Jeremy,
Cant say I know the guy. I bought the plane in Jan 07 from Dave Beltz. He used it for a glider tug for the last 20+ years. I was lucky enough to get it from him and keep it at the same airport. Been putting some work into it and cleaning it up and she really looks good. As a preformer , gotta see it to believe it. Most dont believe me on what the vsi shows till I take em for a ride. That thing goes !

[TomD]

Ok, got to the bottom of the blistering paint.

When the cowl was painted back in 2000 evidently the painter put a coat of gray primer down and then for whatever reason came back with another coat, but either wiped it down with a solvent or did not let the first coat cure.

When I put new baffles on the engine in 2001 I am guessing it increased the heat enough on the top of the cowl to outgas the solvent or whatever.

The cowl blistered between 2001 and 2006, but did not break through.

When we got to one of the blisters w/ a putty knife huge strips of paint came off with a gray layer beneath and a gray layer adhering to the upper paint layers.

Where the recent blisters had been sanded down to fiberglass the paint was stuck really well.

Took it down to fiberglass resanded primed and painted.

Just another adventure in the wonderful world of jack leg mechanics.

TD

[BudG]

Hey Tom,

Bud here. Was that the paint job done down in Oregon that you just got back?

CAVU2U
BUDG

[TomD]

Evergreen Aviation Services corrected the screw up of a previous painter, and fabric guy.

The work at Evergreen was top notch.

TD

[BudG]

Hey Tom,
That's good to know. Boy, you sure have been through the ringer with your paint.
Great weather this past weekend. Saw you out on Sunday. I went up to Arlington because the grass was being mowed at Harvey. Wife and I went to Vashon Yesterday, tall trees, deep grass. See you soon.