The standard thumb strap on an Ariette is too short and rough. Find some soft leather at a craft shop and make one that's comfortable. I've had to do that on several accordions.

That's what I did actually. And the hand strap. Over all the box plays ok. Been working on love bridge waltz.

I've always had trouble with thumb straps.. no matter the length or material.

I solved it by not using one and playing with my thumb on the edge of the fingerboard. Many makers build with a thumb groove.. many players have added one.

I've got it worked out. It was just awkward at first. Working on love bridge waltz.

HI
I switched my thumb strap using velcro.. I use the soft fuzzy side facing to my skin the other side well I just sits there.. The nice thing is, Velcro don't stretch being made of nylon or whatever its made of.. Very strong stable stuff.. Works for me anyway so far..

Johnnie