Repairing/Overhauling a Cowl

[Eddie P]

Hello guys, I have a question regarding the best way to repair an older cowl that has a few years logged and needs some repair in certain areas due to wear and tear, cracks, etc. I've got a very comfortable level of fiberglass experience and feel really comfortable making repairs but since many of you guys might actually have repaired these Vans cowls I'd love to bounce these questions/ideas off the crowd. Please feel free to comment.

It is an RV-4 cowl of 1991 vintage. A few questions:

1) One area of the cowl has a blister that partially makes way for an alternator mounting bracket - it's a very small blister but it is damaged. I wanted to reinforce the blister as it is cracked in the middle of the bulge. I have measured and I can tell one of the alternator mounting brackets has done the damage over time in this tightly cowled engine. I was going to do an external layup after prepping the area to reinforce the blister and then clean up the inside and re-laminate for some strength. My big question is should I use my preferred West Systems epoxy to lay up the structural glass? Do you guys use epoxy based resins to make simple repairs where necessary? I am much more familiar with epoxy resins vs. polyester. I know we can lay epoxy resin over polyester but not the other way around for long term durability/adhesion.

2) What weight glass cloth do you guys do small repairs with? I was thinking a few layers of 2 ounce cloth or perhaps a couple 4oz layers. Then clean up the inside of the new blister structure (remove the old stuff where needed) and laminate with another layer on the interior for additional strength.
3) Are these 1991 vintage cowls all Polyester Resin?
4) I can't tell if it's a honeycomb structure cowl, but I suspect it is not. Anyone know the easiest way to tell?

In some limited areas on the exterior paint, there are cracks that appear to be the original filler that has cracked (near the engine inlet areas) or possibly just very thick paint. I was going to sand that area down, re fill as needed and re prime prior to the paint finish. Looking at the back side of the cowl it is structurally sound so it is just surface wear and tear. As far as fillers go I'm interested in what brand you guys have has good success with for filling pin holes, etc? I have a few brands out there that I've used but I'd like to know what ones have stood the test of time on a Vans cowl.

Of course, the first order of business is to de grease the cowl interior and exterior. Over time I can tell there has been some small level oil intrusion (or at least grease and grime) perhaps in some areas and that needs to be addressed first.

Very much interested in hearing about any other thoughts. Thanks in advance.

 

[rvbuilder2002]

3) Are these 1991 vintage cowls all Polyester Resin?
4) I can't tell if it's a honeycomb structure cowl, but I suspect it is not. Anyone know the easiest way to tell?

Prepreg epoxy cowls didn't become available for RV's until sometime later in 1996.

The easiest way to confirm polyester resin is to sand on it and then smell. If the sanded spot smells like poly resin, then it is a polyester cowl.

 

[Eddie P]

OK, then that's pretty easy. I know what you mean about the sanded material and the smell for polyester resin. Looking at the cowl it sure seems like the older heavy style basic layup cowl. Thanks.

Anyone else care to share their favorite resin materials and fillers for filling gaps and making repairs? I know I can use a variety of materials I have on hand I just want to make sure there isn't anything out there that would be better suited for the temperature extremes and vibrations a cowl will likely face over several years.

 

[rbibb]

Its polyester. If it was honeycomb you could tell from the inside.

The cracking is probably the gel coat cracking. I just used whatever glass you find at the autoparts store (or even fiberglass mat) to reinforce those areas that need repair. While not the lightest stuff in the world good old Bondo will suffice to fill in as needed. You won't need much so weight shouldn't be a concern.

The epoxy cowls are noticeably lighter but probably not worth the cost to replace it. If you have a 4" prop extension it is the old style "long" cowl. I don't think they ever made them in epoxy.

I've had both kinds on my RV-4 - either one is fine.

I'd probably make all the repairs and repaint the whole thing if it was me. Good time to upgrade to fasteners instead of hinges per Dan Horton's posting if you prefer. Great thing about fiberglass is you can fix almost any problem you might have. Bad thing about fiberglass is you will hate every minute of it.

 

[Eddie P]

Yeah on the surface cracks I can tell it's the paint and perhaps some gel coat but I can also see some "red spot putty" under one of the larger paint cracks. The red spot putty appears to be originally applied a little ticker than I am used to seeing since red spot putty is typically only used to fill pin holes or very small imperfections. I suspect the area may have cracked long ago due to too much spot putty being used in this area.

Speaking of red spot putty, do you guys use it to fill pin holes as needed even though it is not really an epoxy filler when doing epoxy repairs? I've used this myself many times for just small pinhole repairs but not sure on a cowl with heat, vibration, cold, etc. Pin holes can be difficult to fill with much anything else in my experience but I'd like to know what you guys use.

 

[DanH]

I'd sand away all the paint, then sand a depression into the gel coat everywhere you find a crack. With the gel coat and all the old **** filler gone from the cracked spots you'll be able to see if you really need repair of the primary fiberglass. Do glass repairs as necessary (scarfs are best), then do any final surfacing work with a good grade of polyester filler where you had removed gel coat. Evercoat Rage Gold is popular.

Speaking of red spot putty, do you guys use it to fill pin holes as needed even though it is not really an epoxy filler when doing epoxy repairs? I've used this myself many times for just small pinhole repairs but not sure on a cowl with heat, vibration, cold, etc. Pin holes can be difficult to fill with much anything else in my experience but I'd like to know what you guys use.

I prefer neat epoxy for primary pinhole and scratch filling, sanded slick and dull, then shot with an epoxy primer. The primer will accent any pinhole or divot you missed. If the previous steps were done right there will be very few (perhaps one pinhole per sq ft, usually less). For those I will wipe spot putty into the pinhole, then sand away every trace not down in the hole. It's just the last quick fix before painting.

 

[Eddie P]

Dan thank you for the tips. VERY much appreciated.

One follow up if you get around to seeing this again is: Assuming I use epoxy resin to make repairs (west systems as I have a relatively fresh supply already on hand) - I would love to use the Evercoat products as filler as I've had a lot of good luck with this in the past. But is there any threat of delamination/speparation of the Evercoat used on top of an epoxy repair, or is it just fine? Perhaps I am assuming but is Evercoat Rage Gold a polyester filler? So if it works I'm all about doing it myself!

One reason I am interested in using epoxy resin for the repairs is the ready availability in my on hand stock but also there have been previously done repairs and I suspect those are epoxy resin cloth repairs to the original polyester resin cowl.

Thanks for the scarf repair tip, that's indeed what I also had in mind.

 

[DanH]

Assuming I use epoxy resin to make repairs (west systems as I have a relatively fresh supply already on hand) - I would love to use the Evercoat products as filler as I've had a lot of good luck with this in the past. But is there any threat of delamination/speparation of the Evercoat used on top of an epoxy repair, or is it just fine? Perhaps I am assuming but is Evercoat Rage Gold a polyester filler?

You can do epoxy/glass layups on a polyester/glass base material.

As for Rage over epoxy, that's a lot like primer debates or politics. Personally I do not use any polyester fillers over epoxy, with the exception of the previously noted teeny tiny quantities down inside the random missed pinhole. I have had polyester filler delaminate years after application and ruin in a nice epoxy/glass project. Others claim no issues. Maybe products have changed, maybe not. It's your airplane brother.

 

[Eddie P]

OK, that's what I was thinking as well, thanks for the second opinion. More concerned over the long run on exactly what you mentioned - but this cowl is old and I suspect a new one will probably be in order in another few years due to oil ingress and general erosion in some spots. Perhaps the Evercoat as you mention will be a good option and I'll look into some polyester resin as an optional method to keep it all one chemical type. Cheers-

 

[Eddie P]

Just as an update, I was able to do some good work over a few days and address some areas of concern in the cowl.

I made several scarf joint repairs to some areas of stress cracks and was able to cosmetically fill/repair some wear and tear areas on the cowl. I also made a few lamination repairs to areas that showed evidence of oil soaking over the years. I removed suspect areas of delaminated glass, de-greased the areas thoroughly and cleaned again before a scuff sand and wipe down. Then made the lamination repairs to those areas. All told I am quite happy with the results after a layer of heat insulation tape was applied inside the cowl then a sealing primer coat was put over the repair areas before the conditional inspection was completed, signed off, and the aircraft test flown.

I will likely re-paint much of the lower cowl if not all of it as there was some work done in many places. Besides, I noted the old paint was clearly put on too thick. It must have a good 7 pounds of extra paint on that cowl above and beyond what was needed.

Question for you guys:

1) The cowl has the nice cam lock fasteners. Any of you guys ever take those off? Looks like there is a lock washer, etc.

2) What is the easiest, least destructive way to de mount the cam fasteners so I can sand down the cowl and re paint without scuffing up the nice stainless steel exterior of those fasteners? Then I would re mount them of course over the freshly painted cowl.

Thanks