Tip: The perfect pinhole/weave filler process

bpattonsoa · #1 09-04-2013, 10:57 AM

Drywall mud. Don't laugh, I took a RV-10 cabin and doors from the raw contoured fiberglass to a finished primer coating in about 14 hours. There is one pinhole.

The first thing to understand is that the pinhole fillers are all some sort of liquid that holds talc (or calcite, mica, gypsum) in suspension and will hopefully flow into a pinhole to fill it. This method puts the talc in the hole and then turns it into a hard solid.

First is the contouring stage where the shape is formed. It includes filling with micro around and on the doors to get a good fit with the cabin top and the glass around the windows (if you do it). It is not included in my 14 hours.

Next is to sand the entire thing with #36 or so paper to open up all the pinholes and expose the weave of the glass. Clean the dust well, blow it out of all the crevices, pinholes and weave. You can see it, especially if you have a pink cabin. I used a small wire brush to scrub the white spots while blowing with an air gun.

Next take drywall mud and thin it out slightly, about 5-10% water, you want it loose but not soupy. Put it on with a hard squeegee, pushing hard to rub it in until it goes into all the holes. Sometimes it takes several passes until you see the mud filling the pinholes and weave. You don't want to leave a lot of extra mud on the surface. Ridges are easy to sand off later. This process is easier to do than explain.

After the mud is dry sand it with a block and #180 or so paper. I use a medium hard foam block. Remove all the surface drywall mud until all that is left is the dots and lines that are filling the pinholes and weave. If you over sand, add more mud and do it again.

Clean off the dust. Do not blow it with an air gun!! You can blow the talc out of the weave at this stage. I did it with a large brush and duster.

Now it is time to turn the talc into a hard filler. Mix some epoxy primer. You don't need as much as you would think, I used about 6 oz to do the complete cabin top and doors. Put it on with a squeegee. The talc will adsorb the primer and turn the color of the primer. Try to avoid leaving lines or drops of primer on the surface. I recommend a soft squeegee, window washing type, about 3-4" wide if you can find one.

When it is cured, sand with a soft block and #320 wet/dry used wet. Just sand enough to make it smooth, don't sand down into the original fiberglass. This is very rewarding because you can see the perfect surface, free of weave of pinholes. If there are any, you can go back and put in more drywall mud and repeat. I must have missed my one pinhole at this stage.

Prime with an epoxy primer and it is ready for paint.

This will work for any fiberglass surface. I got the idea from our local fiberglass expert who turns out some amazing paint jobs on his aircraft in a very short period of time. I did not include any discussion of masking or protection of other surfaces, but the time required is included in my 14 hours.

rocketbob · #2 09-05-2013, 05:02 AM

Great tip, no reason why it shouldn't work well.

krw5927 · #3 09-05-2013, 05:30 AM

Bruce, Bob, or anyone else with experience: Does it have to be epoxy primer that's wiped over the cured drywall mud? Or could one use thinned epoxy instead?

I hesitate to choose a primer before I make the "paint myself or professional painter" decision and choose a paint system, as I understand it's best if the primer is from the same line as the top coats.

Greg Arehart · #4 09-05-2013, 07:17 AM

Good idea, but just a clarification: drywall is not made of talc, it is gypsum.

Wondering whether the weight is significant? I would think not, but any idea of how much mud actually is added to the final finish?

Greg

Brantel · #5 09-05-2013, 07:35 AM

I have no idea what Loehle Wonder-Fil is made of but when it is top coated, contrary to the advertisement, only the surface of the powder gets saturated by the coating leaving a powder bomb underneath.

I suspect gypsum will do the same thing.

krw5927 · #6 09-05-2013, 07:37 AM

While drywall is made of gypsum, joint compound seems to be a mixture of things which may or may not include gypsum or talc.

http://en.wikipedia.org/wiki/Joint_compound

Probably doesn't mean it's not a good pinhole filler. Just different perhaps from what the OP described.

DanH · #7 09-05-2013, 07:59 AM

Hmmm....$150K for an RV-10, $4-$10K to paint it, and drywall mud as a filler?

Let's assume you really like the concept of wiped/squeegeed talc pinhole filler cured by the addition of subsequent coatings. If so, perhaps it would be better to use a filler carefully engineered for the job.

Akzo Nobel 28C1 is a qualified product at all the aerospace majors (Bell Helicopter, Boeing, EADS, Lockheed Martin, etc) and costs a whopping $65 per quart plus shipping.

http://www.anac.com/TechnicalDatashe...Nobel_28c1.pdf

sf3543 · #8 09-05-2013, 08:38 AM

Dan
On the following...
"Akzo Nobel 28C1 is a qualified product at all the aerospace majors (Bell Helicopter, Boeing, EADS, Lockheed Martin, etc) and costs a whopping $65 per quart plus shipping."
Where do you purchase it? I saw a few places selling gallons at over $200 but no Qts. I'd like to try it but I don't need a gallon.
Thanks,

bpattonsoa · #9 09-05-2013, 08:43 AM

You do not want to use drywall mud as a bulk filler. That is the job of micro, bondo, etc. This process starts after the contouring is done. The mud is very porous but I would not want it to be thick. I doubt if any spot was more than 1/32"" thick. It looks like flecks or small dots.

Dried mud is not heavy when dry and I doubt if more than 5% of it is left after sanding. You want to sand until you see mostly the fiberglass and very small bits of dried mud in the pinholes and weave.

I tried a small area using unthinned West system epoxy over the mud. It worked but was much harder to sand. I suspect thinned epoxy would work well.

Not wanting to start a primer war, but I would say that a well curred epoxy primer does not care whose product goes on top. I am planning to fly with this in primer and then sand it when I paint.

rocketbob · #10 09-05-2013, 08:52 AM

Quote:

Originally Posted by DanH

Hmmm....$150K for an RV-10, $4-$10K to paint it, and drywall mud as a filler?

If its proven to work well by an experienced painter, why not. Talc is used widely in polyurethane paints, gypsum is just a slightly harder mineral. I'll wager to bet that $65 can of primer consists of a pretty high percentage of talc.

The other thing to consider is its impossible to clean most pinholes, so many impurities get trapped in them during the finishing process that never really causes painting issues.