How do I stop my backstitch from puckering on painted fabric?

The pucker is mostly the paint, not your tension. When you paint fabric with a watercolor wash, especally on something as dense as cotton canvas, the binder and pigment settle into the fibers and basicly set them in place. The fabric loses its drape and can't absorb the slight shifting that normal stitching needs. So every stitch pulls against a stiff patch that won't move with the thread, and you get little gathers around the letters. Two strands on a 24 is fine, that's not your issue. Going slower won't fix it either, though slowing down does help you notice when the fabric starts to gather before it's too late to undo. Some practical things that help: use a shorter stitch length so each stitch pulls less fabric, and work with a light hoop or none at all depending on which makes the fabric sit flatter. Some people score the back of stiffened fabric gently with a needle to break up the paint layer a bit before stitching, which gives the fibers a little flex back. Test that on a corner first though, since it can leave faint lines. A sharper needle also helps, it pushes between fibers instead of dragging through them.

Stitching first and painting after is honestly the better order for this kind of piece. The paint then locks the stitches in place and the thread becomes part of the surface instead of fighting it. The tradeoff is the paint can pool around the threads and change the look of the letters slightly, and you have to be careful not to gunk up the floss. But for letters that need to stay crisp over a wash, it ussually beats fighting stiffened fabric.

For washing, spot cleaning is really the only safe route with watercolor on canvas. Most watercolors are not truly washfast once dried, they can re_activate with water and bleed into the fabric around the painted area. A light mist test on a corner will show you fast if the color moves. If it does hold up to a damp test, still hand wash cold, no agitation, and dry flat. If it bleeds even a little, keep it to spot cleaning with a barely damp cloth and accept that a handled piece will show some wear over time.

I did a similar piece last year and the thing that saved me was warming the fabric before stitching. I hit the painted area with an iron on medium through a cloth and it softened things up just enough that the needle stopped fighting me. Not perfect, but the puckers got way smaller. Also second whoever said shorter stitches, that made a bigger difference than anything else I tried.

For the washing worry, I'd acutally test a corner now before you get too deep into it. If the color moves with a damp cotton swab you still have time to rethink the whole plan or redo the wash with something meant for fabric.

I can't get behind the stitching first advice from the first reply. If you paint over finished backstitch, that wash goes right onto the floss, and dark red cotton soaks up watercolor like a sponge. You'd end up tinting your letters and the paint would pool around every little stitch bump, which is the opposite of crisp. The raised thread also means you can't get an even coat, so you'd be brushing paint on in two passes and making a mess of the edges. Stitch through the paint if you have to, but paint over thread on a lettering piece and you'll regret it. Curious if anyone here actually tried that order and got clean letters, because I didn't.

Something that's helped me on painted panels is backing the fabric before I stitch. A lightweight fusible interfacing on the reverse gives the stitches something to grab into instead of just the stiffened surface, and the puckers lie down a lot better. I did a 14-word banner on painted duck cloth last fall and skipped the interfacing on the first panel, regretted it the whole way through, added it to the other two and the letters sat flat. Also, since yours is meant to be handled, that backing has the bonus of hiding the thread carries on the back so nothing snags.