Hi, smeagol, acetone is a solvent; the application most people would be familiar with would be as the main ingredient in nail polish remover. A lot of guys buy it at the grocery store or drugstore in that form. If you were to buy it as nail polish remover, remember that it's a dilute solution (diluted with water), and you might want to look for non-scented brands. As a manly man, I buy mine at the hardware store *grunt, grunt, grunt*
When I make a paste of putty and acetone on the palette, I put a little bit of putty, about a quarter of how much toothpaste you might squeeze out of a tube onto a toothbrush, and use an eye dropper to drop acetone onto it. Then I stir it with a toothpick until I get it to a consistency that I want, and brush it on the seam.
The other way I use it is to apply the putty to the seam, work it into the seam with a toothpick or knife blade, and then to dip a cotton swab in the acetone and run it over the puttied join to smooth it out and remove the excess. What's left is in the seam.
I use the cotton swabs that have the rolled paper shafts, not Q-tips, whose plastic shafts the solvent would melt.
I have some pics of my Godzilla posted here somewhere, but I'll put up some newer ones. I didn't make the seams disappear, but I was satisfied with the look, when I was done. For me, the fun was to take a more serious stab at this classic Aurora kit than I did when I was a kid, when glue meant using Duco Household Cement, and paints were only Testor's gloss enamels, if I used paint at all. A trip down Memory Lane....