I find it really tricky to stop the soot from the cut edges marring the top coat of paint. Even when I wipe over first, and keep my fingers well away, there’s still some smudges
You aren’t using spray paint? Baltic birch doesn’t make soot… nor does acrylic, and mdf, and lots of other materials. Not sure what ‘greyboard’ is, but if soot if a problem you’ll probably be happier using a more appropriate material, even if it’s a bit more expensive.
It’s a trade off - I apply the paint with a hard roller so the sides of the pieces stay dark. If I used spray paint once assembled, overspray would ruin the contrast.
I think US chipboard is UK greyboard - a grey cardboard. And yes, I’d like to use birch ply or similar, but I’m hoping to sell at a craft fair and I’m worried the costs won’t get things sold.
Baltic Birch, shopped correctly in the US, is very affordable. I get a 5’x5’ sheet of it for about $14 US, which yields 15 full GF sheets. That works out to under $1 ($0.56, to be exact) per square foot. Assuming your mandalas are about 11"x11", that’s $3-ish for a 6 layer mandala, and that’s if you make zero effort to optimize the free floating parts. I’d imagine the costs are different in the UK, so you’d have to tell us.
I get my birch from Hobarts in the UK. A sheet of birch cut 500x300 is pretty cheap. There is some discount for the number you buy. Might be worth checking out for you.