Recently I found a black acrylic paint with excellent properties. It has a matt finish with very low reflectivity at low incidence angles in red and near-IR light and it is fairly robust, does not mark if touched or scraped and is waterproof. And relatively non-toxic. Maybe not as good as Black 3.0 but I don’t have access to that.
French art company Sennelier produce high quality paints that contain very high pigment content. Their Mars Black (Noir de Mars) 759 acrylic paint uses nanoparticle black iron oxide pigment (PBk11). It gives a matt finish and brushes clean up in water. It is sold in a small 60mL (2 fl/oz) plastic bag. One bag was enough for me to paint the inside of two C925s, a dew shield and all the small baffles. I applied the paint with a 12” artist brush, dappling to give a slightly rough finish. It has very nice properties as a paint. It adheres well to painted surfaces (i.e. original black paint), black anodized aluminium and PVC plastic. Raw metal surfaces require a suitable primer, but it can coat un-primed ss bolt heads and nuts. It is very thick and doesn’t run at all. It dries in 30 min, but seems to come to its darkest after a few hours.
I used a modified Canon 450D that is sensitive to IR, fitted with an Astronomik 642nm BP filter (band pass 642-840nm), so that I could assess reflectivity at low incidence angles in red and near-IR light, covering wavelengths important for deep-sky, lunar and planetary imaging. Some surprises: the black anodised aluminium used in small adapters etc is highly reflective! Black paper is also not especially good. Regular black paints are not good. Theatre paint is not so good. The original paint finishes in my three Celestrons and my Meade 16” were not especially good. Black velvet is very good. Black sharpie ink is reflective.
Looking square on (90 deg), Sennelier’s Mars 759 acrylic black is no better than a typical black paint by eye but at low incidence angle you can see the difference. The IR sensitive camera shows a big difference.
The first images (iPhone) of test-painted aluminium blocks are captured outside on an overcast day at 90deg, ~45deg and ~15deg angles on a black cardboard base. You can see the Mars black is superior at low angles. The big surprise is in deep red/near IR. In the final image I half painted a black anodised M42 T-spacer. You can see the anodising is useless.
After I painted the inside of my hyperstar CPC 925 (f/2.3), I was surprised that shining a white light torch onto the calibration bolts hence onto the corrector plate (originals replaced by ss allen head bolts) did not wash out the image on camera (red light). EAA at f/2.3 appears to have improved contrast. The inside of the telescope tube certainly looks much darker than with the original paint.
Anyway, that is what I found. I don’t like the idea of cementing paper or cork or sawdust or sand, etc into the telescope and I wanted a robust black surface that won’t scuff so I can clean the inside if and when required.