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Pinholes vs Craters

Webb

Well Known Member
Sponsor
Not only the battle but the war was won with my pink Pepto Bismol cowl from HE.. and as my solemn word, I will never build a glass airplane. For those that have one of these pink atrocities, I thought I would share a few things I learned in hopes it will be of help in the same way VAF has helped me on other projects.

Since this cowl has a honeycomb interior, it has horrible pinholes and depressions I referred to as craters in the honeycomb section. I used UV Smooth Prime on other fiberglass parts and it did a good job filling pinholes, I thought it would work fine on the cowl. It did on the periphery where there was no honeycomb. After sanding the primer off, you could still feel the depressions in the honeycomb section with your fingers. The pin holes were filled, but they were still depressions.

The remedy was to follow Dan Horton‘s advice. Using a foam roller, I rolled three coats of neat epoxy and sanded until the orange peel was gone. The surface is now perfectly smooth and free of any depressions/craters. Looking back, I should have eliminated the Smooth Prime and only have done the epoxy step which would have saved a lot of time.

IMHO sanded neat epoxy isn’t the easiest or the fastest way to tackle pinholes, however it produces the best results. In this particular case, I could’ve eliminated the primer and cut my time in half.
 

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Looking back, I should have eliminated the Smooth Prime and only have done the epoxy step which would have saved a lot of time.
This!
I did Smooth Prime approach on my second build (RV-10) as my Lancair IV neighbor strongly recommended it. It more than doubled the work compared to neat epoxy, sand then PPG (Omni) high build primer as the sand coat before prime/topcoat.

Fill the craters with micro and block sand before anything else.

Warning - NEVER wet sand Smooth Prime!

Carl
 
Finished the cowl. The rope…………that was in case the cowl won………

I like smooth prime on non honeycomb fiberglass like a fairing because it is easy to roll on and then sand because the surface is already fairly level/smooth but it is definitely not made for surfaces with depressions.
 

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The earlier green honeycomb cowl that came in my kit was also full of small pits and craters. I put on a first coat of wet spoxy micro and squegeed it in then sanded and brushed on a second coat of straight epoxy thinned per the Vans construction manual. After sanding this a third coat of epoxy primer was sprayed on and there were no pinholes. It was quite a bit of work and took several days overall to get the cowl ready for painting.


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The earlier green honeycomb cowl that came in my kit was also full of small pits and craters. I put on a first coat of wet spoxy micro and squegeed it in then sanded and brushed on a second coat of straight epoxy thinned per the Vans construction manual. After sanding this a third coat of epoxy primer was sprayed on and there were no pinholes. It was quite a bit of work and took several days overall to get the cowl ready for painting.

Don't thin epoxy. It's a physical properties risk with no benefit, and makes it harder to build a suitable thickness.
 
I didn't invent the idea. It's straight from the composites community. Best known early example was probably Cory Bird's Symmetry, but there are plenty more in the EZ world.


Seals the surface with a moderately hard shell which block sands flat.

Speaking of which...I'd much rather have a green or pink cowl than the later gel coated gray cowl.

Bruce, there's a sticky in the fiberglass forum.
 
Don't thin epoxy. It's a physical properties risk with no benefit, and makes it harder to build a suitable thickness.
Understood; in this situation the acetone flashes off in a few seconds, leaving a thin layer of neat epoxy to seal the cowl. The benefit of thinning 50:50 as per the Vans construction manual is that a very thin layer can be applied easily and quickly.

I would not thin epoxy in any other situation e.g. in a fibreglass layup or in a fairing filler etc., in alignment with the West Systems technical documentation.
 
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