CharlieC and others,
I tried mixing acrylic colour (tip from Swinger) and quick-drying white glue (thicker basically). Gives you really nice blobs which stay inflated. Will definitely become a part of my repetoire from now on! (Finally found a use for the quick-dry white glue; haven't used it for anything up til now.)
Applied it with fine brush. I tried a small syringe, but the mixture really doesn't suck up that well, and I didn't want to prepare enough of a solution to fill the syringe from the backside.
Mixing with water colour, or water colour pencils tend to make the mixture too diluted, and it doesn't blob as well.
I used my mixture for small knob handles, but it should work fine for big rivets too. They will be even more blob-like than Jim's photo shows (although his blobs are probably just perfect for big rivets, which, after all, are not that bulbuous).
Knobs, blobs, blob-like, and bulbuous, saying it that many times in such a short space make your mouth kind of thicken up...
Leif