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Engine cowl blues

Janekom

Janekom

Well Known Member
Patron
We have treated the cowls with resin/acetone mix as per Vans, but after we have applied the first primer coat we saw that there were thousands of holes. It appears to be form the lack of resin due to the way the cowls are manufactured.
Has anyone out there had it so bad?
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Janekom said:
We have treated the cowls with resin/acetone mix as per Vans, but after we have applied the first primer coat we saw that there were thousands of holes. It appears to be form the lack of resin due to the way the cowls are manufactured.
Has anyone out there had it so bad?
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Welcome to the wonderful world of composites. Getting resin to fully wet out the mold surface during manufacture is almost impossible unless you are prepared to accept the weight penalty of a very resin rich surface, especially with fabrics. Painted, hight pressure autoclave molded carbon fiber Corvette Z06 fenders use unidirectional material to help minimize the effect but even they still need some hand filling and sanding. Van's molders are far less technical than GM's so the only solution is to prime and fill and prime and fill until you are happy.

Jim Sharkey
 
Janekom,

What you are looking at are what we call "pinholes", and they are the bane of an RV-builder's existance! You need to fill and sand, fill and sand numerous times to get rid of them. In my case, I used Van's resin/acetone technique, and it took about 4 coats if I recall correctly. Many more experienced folks use more sophisticated fillers that are probably far superior. Search for "pin holes", and you should get quite a few threads to read...

Paul
 
Thanks guys! This is the second RV I am building and the first one was not nearly this bad. Paul - this is more like the back side of Mars than a pinhole;)
I know it will be a lot of hard work. Just thought this one is really bad.
 
Jan,

I hear what you are saying. It took me four coats of SuperFil to get my cowl to a point where it could be painted. A random orbital sander helped a lot!
 
Guys, guys guys...

This isn't that hard :) - mind you this from a Glass airplane builder.

Oh, and those aren't "pinholes" - Ok some are, but that is affectionatly called "weave" and most of the cowlings that I see after they are painted have that showing thru.

There are 2 ways to fix it. Or probably better a hybrid of both.

a) you need to set the cowling out in the sun before it's painted or primed and let it fully cure. Let the temps get it good a hot (not over 220 degrees tho). Do this on a number of consecutive days. Ya see, epoxy and glass shrink over time when they are new. and until you get it all cured, even if you fill,prime, paint, it will show the weave 2 years down the road.

b) Once you have "A" done above and you feel it's good a cured (there is no specific way to tell, I'd say 30 days of in the sun wouldn't be bad, so plan this early in the build). Then mix yourself some "slurry" of microballoons and epoxy, it still needs to be runny, not the water type runny, but more like cold oil, or honey. Then take one of those Bondo type squeegies and "pull" a layer of it all over the cowling. You are looking to get it about 1/32" or less of an inch thick and a heat gun will make it work much easier. Try to make this as smooth as possible as you'll be sanding most of it off. Now let that cure and the longer the better. Then sand it smooth and shoot a layer of primer of a differing color and sand that smooth. See where you missed or sanded too much and fix as needed. Now you should be ready for a high-build primer, sand and paint.

c) in place of 2, you can try this step... I'd say start on a test area to see how it works first. Use of the right kind of epoxy is key to this method. West seems to work fine.

d) mix up some just plain old epoxy and curative. if you can do this in a hot environment or warm (don't heat) the mix. Now pour in on the surface and pull it all over with that same bondo squeegie. You don't want it thick or it will run, it will also "self" level as needed if warmed right. Wait until it's "tack" cured, and do this one more time over the top of the tack cured version. Then let both cure fully and sand. Check for pinholes on a spot and repeat as needed. The last coat if done right will be so shinny that if you put a light against it, you swear it looks like "glass". Now sand that shine off and put on a coat of primer, sand and put on some guide coat and see how it looks... finish with paint as you need.

There simply is no way to shorten the time, but you can shorten the steps. I really know of no-one that's building composites that fill, sand, fill, sand, etc a bunch of times and then repeat the same with primer.

Hope this helps.
 
Janekom said:
Thanks guys! This is the second RV I am building and the first one was not nearly this bad. Paul - this is more like the back side of Mars than a pinhole;)
I know it will be a lot of hard work. Just thought this one is really bad.
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For the last few years the cowl moldings have been made from a vacuum bagged prepreg honeycomb sandwich. It's very light and strong but uses the minimum of resin to achieve optimum structural properties. There isn't a lot of extra resin floating around to fill the gaps in the fabric weave. The older cowls used a heavy gel coat on the tool surface and a heavy, resin rich wet layup. It's a structural performance, cosmetic finish trade off.
Jim Sharkey
 
RE:Sheet Rock MUD "WHAT"

Jan

I feel your pain. I am right in the middle of the fiberglass fun and games. I have found that finishing off fiberglass is a lot like putting the mud to sheet rock .:eek: Many coats (especially for a novice) with sanding in between each coat. Then shoot the primer and check for any flaws....ya, you got it more mud will probably be needed ...in our case more fill.....reprime.....and ......will you get the drill......There are no real good short cuts..but..there are new products that make the process easier in the cure time and sanding time....Check out the archives and the net and you will be able to find your poison of choice.

All the extra work will pay off as the folks will wooo and haaaa at how great you are a fiberglass. :D

Good luck and keep us informed............

Frank @ 1l8 ... RV7A .....last 994 details
 
Hi all, this is just to report my progress.
After two and a half days of hard work I am happy with the end result.
I know everyone has his/her own secret recipe, but I would like to share mine with the VAF family.

1) Thoroughly clean the cowl with soap and water.
2) Paint on resin, hardener thinned with acetone as per Vans instructions.
3) Lightly sand down. At this stage you will think - I don't have any pinholes and weaves. What are these guys talking about?
4) Clean properly and apply a healthy coat of primer. Here in SA we use a @K filler primer by Plascon.
5) Flat the primer with 220 to 360 grit paper by hand or wit an orbital sander.
6) Here is an important step. Take your air nozzle and thoroughly blow off all the dust going slowly and not missing any part. To your amazement you will now see thousands of holes and weave marks. Don't panic. You have now actually blown clean all the holes.
7) Use a thin spot putty ( here I have used Standox 1k body fine ) and a carpet knife blade or similar to really try and squeeze the putty into the holes. The idea is not to have lumps of putty that you will battle to sand off again.
7) Flat again by hand or orbital sander.
8) Blow dust out of all the holes again - don't rush and do a thorough job. I found that this part is best done outside in direct sunlight ( or if you have spot light it should be ok ) in order to turn the cowl around and at all possible angles. Just when you think you've got all the little *******s, another few will show up at another angle.
9) Apply thin body putty again with your blade.
10) Flat again - this time with a 400 grit paper and do it with a soft sanding block by hand.
11) Clean and degrease again.
12) Apply your final paint.

The end result came out quite well and was confirmed by the usual number of "inspectors" ;) one have while you are building. See the picture below.

Hopefully this will help one or two fellow builders out there.
Jan.

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nice work

Looks great. I keep looking at my pink cowl....it seems like its impossible to ever look as nice as yours. Wanna switch.

Thanks for the tips, will follow your guide.
 
Specialty Pinhole putties from 3M

I wonder if anyone has used any of these 3M products made especially for Pinhole filling. In particular, look at the Pirahana stuff?


http://solutions.3m.com/wps/portal/3M/en_US/3MAutomotive/Aftermarket/Products/Product-Catalog/?PC_7_RJH9U5230GE3E02LECFTDQCEK3_nid=GSYDRNV8J5beVBF19DZVHXgl

The Premium Finishing Putty in the can looks a lot like might be similar to the rub on stuff Lohle Aviation offers at $49 a quart.

Anybody used any of these?

Thanks Bill S
7a
 
BillSchlatterer said:
I wonder if anyone has used any of these 3M products made especially for Pinhole filling. In particular, look at the Pirahana stuff?


http://solutions.3m.com/wps/portal/3M/en_US/3MAutomotive/Aftermarket/Products/Product-Catalog/?PC_7_RJH9U5230GE3E02LECFTDQCEK3_nid=GSYDRNV8J5beVBF19DZVHXgl

The Premium Finishing Putty in the can looks a lot like might be similar to the rub on stuff Lohle Aviation offers at $49 a quart.

Anybody used any of these?

Thanks Bill S
7a
Click to expand...

Most auto body shops have similar polyester products. They cure quickly but eventually shrink. If you don't want to see the weave in your cowl 5 years from now, go with an epoxy based filler. Sure, it'll take longer, but you'll be happier over the long term.
 
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