Questions about filler coats on epoxy-carbon constructions

[merl]

Just thinking about filler coats (a.k.a hot coat) on epoxy/carbon boards: what are the options? My understanding is that the standard choices are (1) classic brushed-on epoxy, which works best if it is an epoxy especially made for finishing; this is needed for a cosmetic carbon-look (2) a wet mix of epoxy and glass bubbles (q-cell) for the classic race-board I-can-see-you-have-carbon-under-the-white-stuff look.

With both of these one might finish with a UV-protecting layer of spray lacquer unless in (1) you use a special UV-stable epoxy. Does that summarise the standard approaches?

What about an alternative: using a car-body filler spray as the filler? This is designed to fill, and to be really easy to sand back.
https://www.biltema.se/en-se/car-care/c ... 2000028149

Thoughts?

[BWD]

Filling the weave serves a structural purpose as well as cosmetic/hydrodynamic, making the lamination resistant to dings and buckling. A first fill coat of straight epoxy is good when making light laminations of just a few layers of cloth (carbon or otherwise). Next layers, sanding or finish, paint, etc. are better for fillers, in my opinion at least.

[TomW]

Just thinking about filler coats (a.k.a hot coat) on epoxy/carbon boards: what are the options? My understanding is that the standard choices are (1) classic brushed-on epoxy, which works best if it is an epoxy especially made for finishing; this is needed for a cosmetic carbon-look (2) a wet mix of epoxy and glass bubbles (q-cell) for the classic race-board I-can-see-you-have-carbon-under-the-white-stuff look.

With both of these one might finish with a UV-protecting layer of spray lacquer unless in (1) you use a special UV-stable epoxy. Does that summarise the standard approaches?

What about an alternative: using a car-body filler spray as the filler? This is designed to fill, and to be really easy to sand back.
https://www.biltema.se/en-se/car-care/c ... 2000028149

Thoughts?

I have tried:
1. 2 component polyurethane filler coat used in car industry. Nasty stuff with fumes that will kill you and stinks up your shop, house and neighborhood. Not recommended, no advantages.
2. Glass balloon mix with epoxy and pigment. Easy to see when to stop sanding, easier to sand. This is primary advantage. It will absorb water if left as is. And there is tendency to get micro air bubbles that have to be filled after sanding. Then I apply normal gloss coat that can either be left glossy or normally sanded to 800 grit for Matt finish. I think it looks better matt. I've done both matt and gloss.
3. Clear epoxy sanding coat using easycomposites.co.uk XCR coating resin. Sand with machine to 180g. Then coat again for gloss. This method requires most skill to not sand through and achieve good gloss.

When I'm lazy I use method 2. This is why a lot of manufacturers use this method. But mass producers usually use a 2 component PU spray finish to save time, eliminating that last epoxy coat. If you have pro level spray box this is fastest. But again, the PU paint is seriously nasty.

[alpaia]

Thanks for sharing yor experience TomW.
For method 3 clear coat Sicomin have a specialized "Top Clear" resin too, it looks really good but pricy :
https://www.boutique-resine-epoxy.fr/re ... -1533.html
It seems to be developped specifically to make a filler and clear coat with surface tension, like the coating resin you use I guess.

[downunder]

It is not pure epoxy we need for perfect finish.

We need a hybrid which is epoxy with acrylic. Easy to sand.

https://www.sanded.com.au/pages/surfset-resins

[alpaia]

Thanks, did not know this hybrid resin. How long have you been using it ?

[downunder]

People are using it for quite some time.

Have a look at this finish:

https://www.facebook.com/bodhitreesurf

If I did not buy 4L recently I would go with this, absolutely. But, all my projects are on hold for now anyway.

The epoxy is not a problem, sanding it off is. That mirror shine comes from sanding, as we know.

[alpaia]

Thanks for the link lovely hollow wooden surf indeed. Yes sanding epoxy is work and also the dreaded "too late you hit the glass" is so frustrating.