Drywall mud pinhole busting: a few questions

I tried the drywall mud pinhole filling process today on a fiberglass test piece. The part didn't turn out perfectly, but I think that's due to my error. The drywall mud itself showed definite potential.

Q1: In the process description, it is suggested to use a "duster" and a large brush to remove the drywall mud dust prior to applying the primer. What type of duster is this---a feather duster or one of those "canned air" dusters? I'm thinking feather.

Q2: After the mud has been sanded, the epoxy primer is applied. Should the dried surface of the primer be smooth or does that happen during the final sanding? I could still see the weave fairly well prior to sanding.

Q3: The part I tested this was flat but had "natural" 7781 weave on the surface that I was working with. My understanding is that in this scenario I should be able to jump straight into the drywall mud process without any other prior filling steps. Does that sound correct?

Thanks for the help,
David

I'm typing on my iPhone from OSHKOSH so I can't be too verbose.

A1. Just wipe the dust off with a tack cloth before priming.

A2 and A3. The mud is used to only fill pin holes. It's not filler. If you want to fill weave, use some micro. Mud will not stick on flat surfaces but it's does a good job of filling those pesky holes.

Q1: In the process description, it is suggested to use a "duster" and a large brush to remove the drywall mud dust prior to applying the primer. What type of duster is this---a feather duster or one of those "canned air" dusters? I'm thinking feather.

The drywall is very soft and can easily be displaced by sanding or blowing it with air. I used a large very soft paint brush.

Before starting with the drywall, the surfaces must be completed, i.e., all the micro and other filling done. Then sand it with something somewhat rough, like 80 grit and final clean with compressed air, blowing all the dust out of the pinholes and weave.

When sanding the drywall, you must use a very soft touch, that may be why the drywall mud came off the fiberglass weave. I prefer a foam block sanding block that comes in various grits, about 180 seems to work well. When done, you should see only small dots of mud in the pinholes and small pieces in the weave.



Q2: After the mud has been sanded, the epoxy primer is applied. Should the dried surface of the primer be smooth or does that happen during the final sanding? I could still see the weave fairly well prior to sanding.

When using a squeegee you will have some ridges and rough spots in the primer. Just get a good coat over the entire surface. Sand it wet with a 320 or so foam block and don't go thru the primer. Very light pressure with the sanding block, just like with the drywall.

Q3: The part I tested this was flat but had "natural" 7781 weave on the surface that I was working with. My understanding is that in this scenario I should be able to jump straight into the drywall mud process without any other prior filling steps. Does that sound correct?

As above, the surface should be ready for the paint process before starting with drywall mud. If the hand applied primer still shows weave after the drywall process, it must be very small. Try another coat of primer over the weave and block sanding it down.

After I do the primer sanding, I spray a coat of primer over the surface. In the case of my -10, I didn't get to painting for a few months so I just lightly sanded that primer to start the paint process.

Here is the result.

That is really sharp Bruce! Fabulous!

Bruce and Randy,

Thanks for setting this fiberglass noob straight. I need all the help I can get at this stage.

Bruce---your airplane looks wonderful!

David