For months, I've had some handy sanding sticks knocking around.
Each is around 3mm wide and on top and bottom, has some abrasive material. Each packs contains a variety of grades of abrasive. The sticks are a sort of foamboard but plenty stiff enough for the job and very pokable into places other abrasives find hard to reach.
Of course, I wanted to blog about them, but couldn't remember where I got them from - not very helpful.
Well, at the weekend, I managed to buy another pack - £2 from Ambis Engineering. I can't see them on the website so they might be a show only product. Worth seeking out though.